Wow you guys have been together for 3 years and have only recently kiss lips? I respect that 'cause not many couples can do that.

Anyway, I seriously don't think there's any problem with sleeping together. However, your girlfriend might want something more than that when you guys are in the same bed, so you need to be PREPARED! It seems like you are really "innocent," so I am gonna give you the details.

1. Get some condoms (don't worry, they won't say nor ask anything). Durex is a good brand.
2. Google on how to use a condom.
3. Use it when you masturbate. You want to make sure you know how to use it before the real show. Premarital pregnancy is a big no-no when you are only 18! If the condom does not burst and your semen does not leak, you should be using it right.
4. MAN UP!

Err... watch some porn if you have no idea how to have sex. I don't think I am allowed to say anything regarding that on the forum, lol...

Good luck and have fun =)