Just yesterday I started my first campaign where I am the DM, and I have 7 players in my campaign (was originally 9, but two couldn't make session times). I have played as a PC before, but it was two or three sessions across multiple campaigns just because I was busy at the time. for my campaign I made a homebrew continent and made my own story, but I had no idea how I would prepare for each session. For the first session, I gave my players a prologue showing them the future, but they don't know it's the future, then brought it back to the present by starting off in a good ol' tavern. after that, I made up the rest of the session as we went, then wrote down notes for what happened so I could tie it into later story. So for example, the players were looking around a temple after taking shelter from a sandstorm, and when asked about the names on the gravestones, I made there be seven headstones, one for each character, and for the name I put the most prominent characteristic of each of the PCs, without telling them of course. I plan to make this a sign of a possible future, especially because the temple they were at was the temple for an illusion/trickster goddess.

Other than that, I have a general idea for the main plot of the story, and I know what the main goal will be, and who the BBEG will be.

Any advice for preparing sessions, or just DMing in general?

First off 7 is a lot of players for a first time DM. Thank God 2 players dropped out because 9 would be almost too much to handle for all but the most experienced dungeon masters.

General advice: have an area or two planned out for the players with a couple of encounters mixed in, but keep it loose. PCs will often surprise you with their ingenuity so it's better to be able to improvise with the situation rather than stay static with what you've planned and railroad them. 

Might I also add that we get a "first time DMing thread, any advice" damn near everyday. If you scroll through the forum you'll likely find a lot of great advice from similar posts.

Thank you! I really wasn't expecting so many people to join, but I didn't want to deny some of them. The first session did really well, none of my reserved players were over-powered by the more talkative ones, and while we haven't done any real combat yet, I am planning for an ambush to happen soon.

I wrote down a recap for the first session that I plan to say at the beginning of the next session, so I can post it here if y'all want a recap of what happened to help give advice.

I think the best piece of advice for the type of game you seem to be going for is not to get too hung up on having a "story" as such. 

Make sure that you clearly understand what the BBEG is up to, how they are going to go about doing it, drop the players some hints about this evil plan as appropriate, and then let them have at it. 

Also - be prepared for the players to completely ignore any and all hints that you drop, and instead become completely obsessed with some insignificant detail.  If this happens - don't panic!  Just let them get on with it for a while, and then try and inject some more hints about the actual plot if things really start to go off the rails. 

If you are running a "theatre of the mind" game, your session prep can be pretty minimal - I'd suggest that at the end of your next session you ask the players what they are going to do next, and then draw up a rough outline for next week's session to help you keep things moving.  If your players don't know what to do next, great - that just gives you free reign to point them in a direction you choose.  I'd also suggest that you prepare in advance at least 5 or 6 encounters you can keep in your back pocket in case you need them - as you get a bit of experience you'll be able to generate them on the fly a lot more easily, but to begin with it's one less moving part to worry about. 

I'm now going to try to expand the canon of DM advice videos beyond linking Matt Colville (blasphemy, I know) - though I should say his videos are great too!

Finally, have fun with it!  The best D&D is when everyone is having a good time, DMs included. 

Thank you, I will definitely start asking them where they'll go, cause me knowing how chaotic they can be, for all I know, they might just get lost, so that'll be great help to set a base for preparing the next session. Thanks for the video recommendation! My Youtube recommended is probably filled with "how to dm" videos now, but they certainly are great help.

MAKE. SURE. TO. TALK. TO. YOUR. PLAYERS. I used all caps because the amount of people who have problems on the forums that havn't talked to their players is mind boggling. I would reccomend setting aside 30 minutes every game to debrief so everyone can talk about what they liked and what they didn't like as a group or individually if they otherwise feel uncomfortable. 

Also, establishing everyones expectations for the game upfront will help alot. If you want to tell an epic story but your players just want to kill cool enemies that is important to work out before the game starts, so you are not trying to weave a story about the world being endangered and politics while your players just want to explode a dragons head and get loot. 

Have some test sessions. This serve to allow the players to get familiar with eachother characters in a way that allows for adjustments to be made without consequence. The players will get familiar with eachother and their own characters both mechanically and personality. Its also a great way for you to get some experience DMing in a very low stakes situation and get a feel for how the group will behave during the actual game.

Lastly, please DO NOT compare yourself to professional entertainment DMs. You are probably not Matt Mercer, you are probably not a trained voice actor, professional writter/story teller, you are probably not trained in improvisational diolauge. You, of your own addition, have not been playing the game for over half of your life. Things are not going to be perfect. Try not to stress about it, as long as you and your players are having fun, you are being the best DM you can be.

It is a GAME not a STORY. Don't get attached to your plot line, the group may not like it and not tell you, or not get the clue. Unless there is a dead PC on ground at the end of the session, don't worry about making mistakes with damage or spells. (Player I just read the shaman stat block it doesn't have a charm spell.  DM. This one did. Or oops. Sorry about that.) 

Write up some notes after the game. I do a three page writeup and post on facebook for my group. Decide how open you want to be with information. I give my players the AC< HP, and status of monsters. Sometimes I have a player track it. Don't worry about the 7 players. After a few sessions, some will drop, some will become casuals, some will want you to dm 24/7.