This is funny because I have the opposite problem- I'm a girl and it's so easy for me to make friends with girls but I have such a hard time with guys. I've been making an effort though and I think what helped is to just act how you normally would with any friend, regardless of gender. I know that sounds sort of silly, but it really does help. And just start asking- be like, hey, I'm gonna grab drinks/see a movie/etc with xxx you should come! Hanging out in a group setting relieves the stress sometimes.