This is a great topic. Sevily, I feel a lot like you do about first person. When it comes to reading, I don't really have a preference as long as it's written well, but as a writer, I usually stick to third person.
That said, I am actually writing a story in first person right now, Curtain Call. I think it's the first novel I've attempted to write in first person in like ten years LOL. I'm enjoying it a lot because it gives it a different feel, but it's also challenging. I'm actually switching between the two main characters' points of view, to show both perspectives, so I've had to not only get the hang of writing in first person, but also writing from two different characters' points of view.
I basically do what Rose and Mare said and try to put myself in their shoes as much as I can. I find it a lot easier with my female character, Cary, because she's not so different from me, so I use a lot of what I would say and do if I were her. Nick is a lot tougher, being that I'm 1) not a guy, 2) not in the music business, and 3) not going through the stuff he is in the story. With him, I just research as much as I can. Sometimes it helps just to watch videos of him in interviews and read his tweets and stuff and pay attention to his mannerisms and the kind of stuff he says and the way he words things. Not that I spend hours watching and studying Bsb videos intently LOL, but now and then they can be inspiring to help you get a better picture in your head. If I can picture the character doing things in the story and imagine them saying the dialogue in my head as I write, it's a lot easier and seems a lot more realistic.
As far as showing not telling, I know it's tempting to have the character tell all about themselves at the beginning... if you can space that out among something actually happening in the story, some action and dialogue and such, then you can still tell what you need to tell without it seeming like "TELLING, NOT SHOWING." SHOW something happening AS you tell the back story, and try to blend the two in a way that seems natural. Easier said than done, I know! I'm definitely not an expert; I've just been muddling through the same thing you are.