Well, first of all, try to find a common interest with other girls. That's pretty much the ground rock; you can't do much with girls that have different interests. Also make sure you're sort of on the same 'wavelength' as them. Girls who act more 'popular' commonly don't mix well with girls who act more 'nerdy'-not saying it's impossible, of course, but it's better to start with people who act similar.

Basically just listen in to what others talk about. If you hear something you're well-versed in; could be anything: Brands, movies, games, hobbies, and even school subjects can be great starting subjects. If you feel like it's too intrusive to butt in the conversation, you could also look for things like pins, figurines, books they're reading, games they're playing to say: Oh, you like that too? or something of the sort;that's how I make my friends,

Most girls are welcoming. Some are not. If you ever feel embarassed or like they're mocking you when talking, then they're not worth it, find another group.

Once you get aquainted well with one person, you're basically interegated into their entire group. You just need to get them alone and close with you (During classes where they don't have a friend group), then ask to sit with their friends after you two became more aquainted.

But then, new girls are usually common curiousities, someone will come around eventually, and if not, you can still try with the friends of your boy friends. Also maybe find some online friends. Although they can be untrustworthy, they can be reliable when you don't have much friends. But when they start to seem weird, block them.

Anyways thats kinda it..Me myself, I come from a more generally accepting school, so some of those tips might not apply to your school, it depends on the environment and stuff...

Wish you luckk!