I don't have any friends who are girls?

Trust me that happens alot. Just remember they can be just as nervous as you, just relax and talk about anything. Start off with something simples, a movie you like and a band you might've heard of. You'll be alright. [:

I used to have somewhat of a similar problem. Try something simple. You're probably thinking "What do girls normally talk about?" and the answer to that is: they talk about normal everyday things too!

Try asking them about what they've been doing recently; any travels, hobbies and interests. How was their weekend? Any plans for their next weekend? It's not that hard, mate. If you're that scared, practice in front of the mirror. You'll be fine :-)

Hello there howdyoudo.

Lol ;) Why can't you have a conversation unless they start it? What makes you feel uncomfortable (or however it is that you are feeling?)

Don't worry about it, I assume you're a guy? Hang around a friend of yours while he is with a girl (be careful not to third wheel though.) If you can get practice talking to a girl in a relaxed environment it really becomes second nature. After a while, you'll be meeting girls left and right.
Hope this helps!

Well, i'm a random girl that talks about anything if you talk to me ;)