What do you say when someone says they like you?

You say, "Thank You!" Sounds like they are giving you a compliment and that deserves recognition.

Another answer:

The answer may depend on how you feel about that person.

If you want to discourage them, you'd probably answer "Thank you. That's very kind of you to say. It's always good to have lots of friends."

If you want to encourage a relationship, it might be better to say "Wow. Thanks. I'm glad you said that, because I like you too."