Whenever I start a conversation I always screw it up by not knowing what to say.

There's more important things to worry about at this age then scoring with girls.

Don't worry, trust me, most girls get nervous when talking to boys. Instead of thinking of them as attractive girls just think about them as if you were just meeting someone new. If you mess up either try to hide your blunder of try to turn it into a joke. Be aware of what you say so you don't get yourself stuck into a corner by reading into what your saying. Cances are you reading into it will be unnecessary because they'll probably assume what your thinking andit will just make the situation awkward. Last thing: don't be afraid to just talk about sports. Believe it or not lots of girls know something about sports but just don't mention your favorite teams unless they mention teams of the NFL or NBA etc. because they might only know what their favorite sport is and why.
Hoped this helped :) :) :)

It's easiest if you have a built-in topic of conversation. Try starting with a question about whatever is going on in one of the classes you share. Ask about a homework assignment, an upcoming test or quiz... Or you can go with talking about school activities that don't involve classes, like "Do you know when the school play is going to be?" If you stick to commonly shared activities, then you won't have to think so hard about what to say; you can say pretty much what you would have said if she were one of your guy friends.

Something else that might help is to borrow the sister of a friend or relative and start by talking to her. Practice getting comfortable talking to a female. And if you can walk down the hall with a girl you already can talk to, then other girls will feel more comfortable. Axdwdrgc is absolutely right. I've answered several questions just like yours coming from 14-year-old girls.

Being 14 isn't easy!