Trying to start a novel

So I have decided I want to try and write a book, I "attempted" this before with other ocs but barely wrote anything over the past four years..I have absolutely no clue on how to start this new one! I'm not an experienced writer, max I've done is fanfiction and school assignments. (Rip to this beautiful poem I'll never see again about vampires that's trapped in my old high school account.) And it's going to be a BL novel, that is boys love. 

I made the ocs for it a little while ago at first they were highs schoolers but I didn't want to write that so now their adults. 

The premise is like this; They knew each other as kids but not childhood friends. They lived in a small town so it was easy to run into each other. They were impartial till they went to the same school when the Love interest became a jack ass and started bullying the mc but not outright he was like sly about it. Y'know when someone is mocking you but you can't prove it, those vibes. So MC is not happy about this obviously, the love interest graduates first or early then moves away while the MC stays in the small town for a while before leaving. Now we're in the future and MC is enrolled in a music school, he took a gap year then came back and that's when he sees the Love interest who's teaching there now. Obviously he's not happy about this and he doesn't trust the guy, Love interest stopped being a jack ass now and is nice enough. There is misunderstandings and stuff plus the mc just distrusts the guy so they're not getting along till later in the story. I want fights and trust issues. That's all I got right now. 

MC is around 21 while love interest is 28, how did they go to the same school as kids? The school had all the grades in one school because I said so and it's a small town and they don't have enough funding shh. 

If you have ever read the insatiable man, I think it's called? that's like a bl manhwa or something. The love interest of that is inspo for this love interest future self. Sorry if that's the wrong name and you guys have no clue what I'm talking about. 

Also gonna include some angst of course, probably some mental illness. I want it to like gloomy or bittersweet vibes but not the whole time, not sure I could achieve that. 

Anyways that's some stuff about it, it's not very outlined and I have no clue how to start. Anything you think I should include?

Should I post a sketch of the characters?