How Do I (10th grade female) Talk to a Senior Guy?

Eh just go for it! My daughter's a freshman and talks to seniors all the time. Be prepared for being rebuffed but it's definitely not out of the question for sophomores and seniors to chat (or date -- I know of several sophomore-senior couples).

As for how, here's a technique she just used (shamelessly stealing from her):

She was acting as "wing lady" for a guy friend of hers who had his eye on a girl at a concert. The girl was wearing a Fall Out Boy t-shirt. Rather than being too literal about it and striking up an easily-overheard conversation about Fall Out Boy, she made a calculation that someone who liked Fall Out Boy would like Dan and Phil, and so she struck up an easily-overheard conversation about THAT. And the girl totally took the bait and joined in. My daughter's guy friend is a freshman, the girl was senior, and the guy friend not only got the senior girl's number but has been has been texting her ever since. So.

Note -- the point is not to use these exact examples, but to similarly find something you think he may be interested in but not so literal as what's on his t-shirt, and strike up a conversation nearby and see what happens.