First person is amazing!
Some rules that have been touched up on, though: In first person, you are essentially becoming that guy who talks about what they did at the party (your setting) last night (in the work):
You DO NOT KNOW what the other characters were thinking (read: if you write what a non first person char is thinking in first person, you're writing a character who assumes what other people are thinking, and is therefore very likely a jackass).
You ABSOLUTELY DO NOT KNOW anything you didn't see (If some story event happened outside your viewpoint char's viewpoint, it doesn't go on the page. If this is absolutely unacceptable, consider whether first person is right for you, or consider changing the viewpoint for that snippet. In my current work, I have, every chapter, a viewpoint indicator saying who my viewpoint character is. It's the same character every time, but it's there.
You know, for certain, how you felt about things. If you don't know how you felt about something, you do and the thing you felt was confusion.
Your beliefs do not feel like beliefs. They feel like reality. (Talk to a trans activist, and they will say trans women/men are women/men, NOT that they believe that. Talk to a follower of Christ, and they will speak about God being everywhere as if it were the most obvious thing on the planet. Talk to a gender abolitionist about why your gender is super important and they'll internally roll their eyes at yet another invocation of a worse-than-useless social construct. See the pattern? Your beliefs, your true beliefs? You won't think of them as beliefs. Your true beliefs will you find by trying to describe The Way Things Are. The things that you say you believe? You want to believe those. You believe that you believe them. But you don't. Because it's not a thing doing a thing, it's believing the thing does the thing.)
You do not in fact have all the answers! (You are allowed to lie about your setting! If your character doesn't know some little-known trick in the magic system, it doesn't exist until that critical moment of protagonist pwnage where you throw a lightning bolt at a guy coated in an armor of magicked water thinking you'll oneshot 'em but it ends up doing nothing because WATER INSULATES! Then they shoot you in the chest with a flintlock while you're staring in disbelief. If your character was lied to by their deepest trust, that lie is true. To them. But you do not lie about your viewpoint character.... Or you do and then you have an interesting dynamic.)
You think to yourself a lot! (First person lets you literally get in your character's head, use it! Showcase their thinking, their beliefs, what makes them tick. You can shift the narrative weight around because it's no longer about the cosmic narrative weight.... it's personal)
As for transitioning from 3rd to 1st, I'm assuming you're used to writing in Third Person Omniscient, where the narrator knows everything about everyone and can just pull things freely. If jumping from that to first person is too daunting, you can try out something called Third Person Limited (Some will say Limited Omniscient, but I have never seen anyone describe a difference between L and LO). The thing with Third Person Limited is, it's still third person, and you may still technically be omniscient- (Oh, that's the difference), but you Stick Very Closely To One Character. For Third Person Limited, you pull on all the thoughts emotions etc of your target character, but you only narrate actions from the others. You know, kinda limiting your scope a bit. And from there, it's a much lesser leap into First Person.
I hope any of this helps, truly I do.
OH! A possible way to think about first vs third person! Third Person, you see the round globe, and everything. First person, you see the flat terrain in immediate eyesight. Let those differences vibe through you while you think a moment, and that might help things.