Yah, striking up a conversation can be hard. P.S. don't do something that'll make you look like a creep (probably shouldn't've said that =P).
Get to know them and then relate. Talk about school. Talk about common interests (which you might not have any, but you might as well hear what she thinks).
If you only see them in the hall, getting to know them can be harder than if you share a class. It's easier to start a relationship wit comments that are relevant.
If anything, just say hi, my name is so-and-so =].
Some of them will probably be surprised or say 'ok...', but the cool ones will respond with something meaningful (like hi, my name is so-and-so). Then after that, ask something or say something that you thought about before (so it doesn't go all akward while you think of something to say) that involves her.
An example would be something along the lines of 'so, you play soccer?', but doesn't have to involve sports (usually involving school or something that just happened to both of you).
Try to be friends with a girl or 2.
Just talk to them like you would boys, except keep it appropriate for mixed company unless it's pretty funny.
Hope I've helped somewhat =].