First time using Birth Control

I am thinking about going on birth control since my boyfriend and I are pretty sexually active. Normally we use condoms. It's totally irresponsible, but we have a bad habit of getting caught up in the moment every once in a while and having unprotected sex, which is definitely scary after the fact. My concern with using birth control is having weight gain...I've heard from friends/other girls that they experienced a lot of weight gain when they've been on the pill and I'm trying to also get into better shape. I've also heard the reverse, where it helps lose weight also, I guess depending on how it reacts with your body. Any suggestions on a specific brand or another form of birth control that would work in the case of unprotected sex?