Taking the Next Step

Ok, I'm 16 years old and a junior in high school, and I've never had a boyfriend. I have boy 'friends' but not a boyfriend.

I can talk to guys all day. I can make penis and fart jokes with the best of them, but I have a problem taking the next step.

I have a few crushes, and I've narrowed it down to the guys I really like, but that still doesn't change the fact that I don't know how to approach a guy to ask him out.

How do you approach a guy? I would really appreciate some advice.