Ask her. I’m assuming you guys are pretty young, so the best thing to do at your age is seriously just talk about it. Trying to “go for it” without knowing where she stands is probably going to lead to you being shot down and supreme awkwardness. Why put yourself (and her) through that? Don’t surprise her. This is new for you both so it’s something you need to talk about.
Ask her if she wants to try something more. Tell her you you want to finger her or go down on her. (If you’re worried you don’t know how to do something, check the sex advice tags because I give many ‘how to’ tips.) Focus this next step on something FOR her. Give before you even think about asking to receive – understand? Don’t ask her for a handjob, ask her if she would want you to do something for her.
If she’s ready for more, she’s likely to offer to return the favor. (Note: I’m not guaranteeing that she will, that’s up to her and what she’s ready for, I’m just suggesting that it’s a possibility.) If you show interest in HER sexual needs, it shows that you actually care about her, not just getting off. It’s a good way to get a girl excited about sexually pleasing you in return. The ice will be broken, curiosity abounds, etc. Just have fun with it and be safe.
If she’s not ready for more, then make your peace with that, and keep being best friends with your hand. π