I'm doing the Starter Kit campaign for the first time and all of my PC's are really immature! One wants to "roll to rizz" all the time, one wants to shout inappropriate insults at all the monsters, and one just wants to give himself the most OP items in the game, make his stats all 18s and basically just mess around with D&D beyond. HELP!

You should discuss with your players about all the things that makes you uncomfortable  or you feel inappropriate. You may want to remind them about the level of seriousness you would like in the game you run and don't want such immature behaviors again, especially if it ruins your fun as DM.

As for ability scores, they are generated during character creation and may increase only when leveling with Ability Score Improvement features as well as some feats or magic items. Since you run Lost Mine of Phandelver, the magic items they will find are already pre-determined so as much as some would like 18's and powerful magic items, just tell them the Starter Set is not really an adventure designed for that.

Good uck with your game and i hope things will get better all around. DMing is an art worth respecting!

Like Plaguescarred said, you should talk and discuss with your players that what they are doing is not right. I recommend establishing some house rules (like to obviously take the game seriously and use common sense) to combat this situation and definitely dish out punishments if they fail to do so. Punishments can quite literally include kicking them out of the game or refusing to let them play the session. 
To just give some ideas:
For the OP items guy: Tell them to fix up his character sheet, else you may or may not have to make a character FOR them and only let them play as it because they can't properly make one themselves.
For the "Rizzing" guy: Trying to flirt with NPCs is fine and all as it can be common in campaigns lol, but definitely tell them to take it more seriously and MAKE SURE TO ADD CONSEQUENCES TO THEIR ACTIONS!!!! (Perhaps the NPC slaps them in the face, or is insulted by them if they roll low.)
For the Inappropriate Insults guy: I cannot emphasise consequences enough, insults could maybe lead to a fight or argument between the NPCs. Maybe the townsguards or so try to arrest them for a bad reputation of "disturbing the peace". You do you!
I believe it's either make them switch up their behaviour or find a more mature group to play with. If they don't take your hard-work seriously, there's no point in wasting your time with them.
Good luck in your next session!

Actually this is very relatable. I tried for while to ignore those things for while but the problems kept coming up.

What I did was I made a little survey  what each player wanted out of the campaign.  I also added things like events or cool combat encounters they wanted to have. 

This helped a good amount, but the more I talked about it to them about the behavior, the less problems we had and more fun we had. 

I still didn’t want to take away all of their fun completely, so occasionally I still let a PC try and seduce people.

idk if this helps, but it definitely worked for my players

Thank you so much for the advice. I will take this into account during the next session. This really helped me know how to deal with these situations, so thanks!

Also, the OP character guy has a hard time staying focused on the campaign. All he does is watch Youtube and play games until his turn. He is never engaged in the combat at all! And when I ask him to focus, he gets angry and says that he's allowed to do whatever he wants. Even harder, he's my friend, so I have a hard time getting him to stay in line because he's so stubborn and thinks that being friends with the DM gives him 'special priveleges'. And he's a really cool guy to hang out with sometimes but he is seriously addicted to screens.

Any advice? I'd appreciate it

To just expand a bit, Tasha’s cauldron of everything has ideas for how to run a Session 0, which is what you need. It’s designed to get everyone on the same page as far as expectations of the kind of game you want to play.

And I’d add to what others have said about the specifics you gave. As the DM, you call for a roll; the players don’t get to decide to roll. And, while it can be fun to try out builds and items, you decide what items they actually get. Some tables like to work with wish lists, others don’t, but either way, you decide. 

I used to ignore session zero but after reading it a lot and and trying it, it actually helped a ton. Maybe try one of those.

It is harder especially if someone is really close to you, but if other people are frustrated and they kindly voice it, it helped a to 

I play with my brother and his friends so he kept on asking me and bribing me to give him good stuff. He also kept saying things while I was talking, which annoyed a lot of other people.

After we had a very short session 0 that was not in the beginning of the campaign, it worked out very well

Cool! I'll try that out. 

Thanks for the advice

I know a player who's very similar to "OP character guy" and still watches Youtube and goes off until his turn. If he thinks he is allowed to do whatever he wants, so be it! You are the GM (god), and if he wants to continue playing the campaign, he has to follow these circumstances, or else he may as well not play.
Some guys can be seriously addicted to screens, and if he doesn't play at all (if you would like to keep the story going with his character), you can play the character for him and he can't do anything about it.
Some players can be extremely immature, and the best way is to warn them and then shut them up if they don't comply. I understand he is your friend, but if he really cares about you and your hard work, he would likely be paying more attention to the story.
Remember, your campaign has rules, and if anyone breaks them, do not hesitate to enforce consequences.
Sometimes it is better to simply kick people who are unwilling to play out than waste your time trying to get the stubborn fools to play.

Quote from Fanny318 >>

Also, the OP character guy has a hard time staying focused on the campaign. All he does is watch Youtube and play games until his turn. He is never engaged in the combat at all! And when I ask him to focus, he gets angry and says that he's allowed to do whatever he wants. Even harder, he's my friend, so I have a hard time getting him to stay in line because he's so stubborn and thinks that being friends with the DM gives him 'special priveleges'. And he's a really cool guy to hang out with sometimes but he is seriously addicted to screens.

Any advice? I'd appreciate it

 You should tell said player that you want more engaged players considering all the effort you're putting into running a game for them, the minimum on their part is to stay focus in order to follow what's going on at the table because there is things occurring off turn that gets easily lost if you're not paying attention. Mention it especially if you need to frequently repeat.

You can go as far as banning any external sources such as TV, cellphones etc when you play if it becomes too much distractions. I did in the past in similar situation to yours.

Being friend with the DM doesn't give any such privileges, the DM deserve respect wether you're strangers, acqainttance or friends. All i can say is that what you experience as DM is not the norm, and is difficult to run enjoyable game with people who have such attitude toward the game and DM. If they don;'t take the matter seriously, you can show them this thread so they can read what other people playing D&D think of this..

TL;DR, but… Simple question: How old are your players?

Because if they’re 12 or under then frankly I don’t know how much there is that you can do. I’ve played with kids of all ages and adults as well, and sometimes you need to indulge in a bit of the chaos with those younger groups. Make sure they understand how the rules work and their place in the world, but let them have some fun as long as you are having fun as well. It’s hard to get a young group to focus on a well-defined storyline, and I should know! I used to be one of those twelve year olds years ago!

Well, all of the players are 13-15 year old teenage boys, so I can sort of understand why they would still be rather immature, but it is kind of frustrating just how immature they are.

Hahahah, I play with 13-15 year olds too since no one else is really interested in DND lol. They can sometimes be a little annoying but with a few corrections they're much better. Good luck with them once again.

I feel you. I can’t rly give this that haven’t been given, but good luck lol, I feel u

I’d honestly kick the player out, regardless of age.  When I played D&D at his age I was completely absorbed, my turn of not.  If he can’t show respect to you and his fellow players, he’s got no business playing at all.  Being angry at you for asking him to focus would be my final straw.

Appellion is right, without being strict with your game they will likely feel like there is nothing wrong with what they are doing.

Alright, I have a plan. I'll try to convince them to respect me a little more and have a session 0. But, if he continues the behaviour, I'll give him a warning and then he's out of the group (for a session or two and then if it still continues, he's out of the group for good.