How do I tell my best friend I like him!? Advice plz?!?

My best friend use to like me and I use to lik me but we never got together and I really regret that. We always talk to each other and never hide anything and I keep not getting his body language. He will sit next to me and b close but Now he's got a gf I hate and she's using him. He won't listen to anyone. Now school is over and I can't stop thinking about him. I don't know what to do. Do I just tell him or what!?!? Do i tell him i like him? Give him options? Ask him out? Plus what about the gf?I'm so confused and I need help! Tell me what u think I should do with details plz!:)