Should I ask my best friend out? I know she's into girls, but I don't know if she'll be into me.

There's nothing wrong in trying. If she says no then it's not the end of the world. If she says yes then try to take the relationship slow at first. あなた have to be sure if it's going to work out first before あなた take any big steps with it to ensure that your friendship won't be damaged because of it.