Guys I need help again 🙏

As a new D&D player, I don’t know all of the rules and spells and stuff, but for some strange reason I find myself being the DM of a campaign. There are certain things I wanna do, (ex: have a crystal seem to mask an area long term, but it can still be found) and generally stuff like that, but I don’t know how much I can break/bend the rules. At that point, it’s not even flavor. I have a friend who’s very knowledgeable on F&D, but she’s in the campaign so I can’t really ask her without exposing the plot. I might be the worst DM, so does anyone have some tips to help?

The crystal covering a whole area seems like the kind of big magic that the rules don't really cover and is something more baked into your setting. The way to do that is to just say "there is a big crystal covering this area".

As a DM, you get to make the rules what you want as long as you explain them to the players beforehand.  So, if want the crystal to mask the area but say for example you don't want the players to be able to find the area with scrying spells, you need to have an explanation for why this particular area is not subject to scrying spells.  As an example, this area could be protected from scrying magic by ancient spells laid down by the Gods and those spells could be emanating from the crystal.  The area could be a demi-plane where the party needs exceptionally powerful magic to locate the area, and the crystal could be the key to finding the area. 

The rules are there so you don't have to make them up.  You can use them or change them to fit your adventure.  If you change them, just have a logical and game-based reason for changing them so the players can understand why there is a change.  You can say, for example, that Clerics can only commune with their Gods, once a day or once a month, even though the player's handbook does not limit the number of times a player can commune with their God.  Your explanation could be your God only communicates during a full moon or something that makes sense for your adventure.

Some tips on generally DMing.  Sketch out the character of a few non-player characters (NPCs) you expect the players to interact with.  Give them some a small back story and unique characteristics.  Keep the backstory to yourself, but it helps you role play what these characters will do in situations and interactions with the players.  For example, Johan Firestarter is half-giant who was abandoned as a babe.  He was taken in by a human family and raised as a human, but his giant size and strength made him stand out and sometimes ostracized by the other boys.  Now that he is an adult, he runs the local tavern and has an affinity for adventurers as long as they are not hunting/battling giants.  Some NPCs that players will usually interact with are a tavern or bar keep, an owner of a general store where they can buy supplies, an owner of a stable where they can buy horses, a temple where they can get healed, and the mayor or elder of the town or village that can provide a plot for the players to solve.   You don't have to have all or any of these types of NPCs, but these are some that you will find in almost any adventure.

Decide what your main villain's objective or goal should be to help establish and guide his actions and plans.  For example, Derrick Evil is a neutral evil wizard.  He has devoted his studies to developing or locating spells that will allow him to capture angelic beings and use their powers to further his aims.  He believes that the world would be better off if there the Gods and their servants did not interfere in the affairs of humankind.  As a child, his parents were extremely religious and required him to attend the local temple.  At this temple, he was repeatedly punished for refusal to follow to rules of the temple.  He thus developed a strong dislike or hatred of structured religion.

Make the players responsible for figuring out their statistics.  The player's handbook has a lot of material on creating characters.  If you let the players develop their characters and coordinate with you on what they would like to do, that leverages their ability to find the information that is pertinent to their own characters, saves you the time from having to figure all of the individual traits of their characters, and gives you some insight into what you need to know for your campaign.  If your players do not have a Druid in their party, you don't necessarily need to know all there is about being a Druid for your first time DMing.  The best way to learn the game is to just start and make sure your players know that you will be learning the rules as the adventure progresses and ask them to help point out rules that will impact their characters as you go along.

Last tip, have fun!  If you make a mistake, admit it, change an outcome to fit the rule, and simply move on.      

Okay, I've got some bad news and I've got some good news. The bad news is that, yes, D&D has a lot of rules. It will take you a long time to learn most of them and you will never know all of them. None of us know all of them.  But here's the good news. D&D is a game about fantasy and imagination, and the very first Rule of D&D is, "Have Fun!"  As long as you and your players are having fun telling a story together, you're doing great. And if anyone ever questions how something works or why something exists, just look them square in the eye and say, "Magic!"   After all, they're already playing D&D, so they're obviously willing to suspend disbelief to a great extent. I mean, that guy's got horns, that lady throws fire from her hands, I'm firing arrows at a dragon! None of this makes sense!

So, about that crystal...  if you want your players to believe in your world, and if you want them to believe that this crystal is a real powerful artifact, the first thing you have to do is... YOU have to believe it! So what is it? How big is it? What color is it? Keep in mind, there are no wrong answers, we're just brainstorming. It must be small enough for a character is lift and carry. Is it a smooth crystal sphere, or is it a broken shard? Oh, that reminds me, if you haven't seen Jim Henson's 1982 movie, "The Dark Crystal", drop everything and go watch it right now. It's an absolute masterpiece. I'll keep typing and you can read the rest of this when you get back.

You say you want the crystal to be able to "mask an area, but still be found". How big of an area? A room, a farm, a forest, a mountain range?  I figure it can't be too big because then everybody in the countryside will notice something's a bit off, so let's say it's in a remote location like deep in an ancient forest. Maybe there's a small lake in the forest and there's a small island in the center of that lake and there's a single old dead tree on that small island, and the crystal is hidden in the hollowed-out trunk of that tree. And the crystal's magic makes the tree and the island invisible, so from the shores of the lake it's just a lake, no island, no tree, no reason to go swimming around looking for anything. But for plot reasons there must be some reason you want the players to be able to find it. So you have to leave clues that lead them toward that forest, and that lake, and out into the water looking for something they can't see, until they reach the island. 

So we know how big it is, and we know where it is, and we know it wants to stay hidden. But we still don't know why. To know why, we have to know WHAT it is. It's obviously powerful, so it must be related to some deity or extra-planar being. And it must have other powers other than the uncanny ability to hide in a tree. And for plot reasons, you need conflict. So there must be bad guys looking for this crystal who want to use its enormous power for their own dastardly plans. So the players have to hurry to find it before the bad guys do, so that'll keep the plot moving forward nicely. But to decide what it is, and to help the characters find it, you'll need to make some sort of connection between the crystal and one or more of your player characters.

Is there a druid in the group? Maybe the crystal is a tear that was shed by a nature goddess. Maybe the bad guys are cutting down or burning forests and the goddess of nature saw that and cried, and one of her tears landed in a tree on an island in a lake in a forest. And that forest is about to get cut down and burned by the bad guys. So maybe the druid gets dreams or premonitions warning them of this. Maybe the goddess speaks through little forest animals to lead the druid to the crystal that will save the forest. So the crystal must have enormous healing powers, and the ability to somehow protect the forest from the bad guys. Maybe whoever holds the crystal can make the trees awaken into treants (that's in the Monster Manual, they're tree creatures like Treebeard from Lord of the Rings), and those treants will help the party fight and defeat the bad guys and save the forest. 

Or maybe there's a wizard in the group. So maybe the crystal is a fallen star. Maybe there have been a lot of shooting stars and meteor showers recently, and the wizard makes a skill check using Nature or Arcana, and they remember an old legend that says stars that fall to Earth contain the powers of the Heavens. So maybe the crystal, besides being a Hide-And-Go-Seek champion, has the ability to manipulate time by shifting when the sun rises and sets, and when the Moon is new or full. That would have massive implications for crops and animal migrations and every bad guy in the world would want to have that kind of power, so the players have to find it first and destroy it.

Or maybe there's a character who believes that the stars are actually the spirits of the dead watching over us at night. So maybe the crystal is a star that fell to Earth and it's the spirit of the grandfather of one of the player characters, and he was wrongly killed and he needs the party's help to get vengeance on the bad guys who murdered him. 

I know, it's a lot. Sorry. I tend to ramble when I'm up this late. All you have to do is make up some plausible explanation of what the crystal is, and once you believe it, your players will believe it too. There are no wrong answers in a fantasy game about imagination. The rules are just there to give us a common framework of how luck and Fate interact with the players. And seriously, go watch, "The Dark Crystal". It'll blow your mind.

Thank all of you guys so much! This was really helpful and honestly validating to hear. Seriously, thank you for all the time and effort you put into those responses. I hope you all have a great day!