I don't think it's that big of a difference, but "vala" is a little closer to "some"
To get "any," "bár" is a little closer:
To answer your question though, yes. Never use "se-" in a positive sentence.
I'm not a native speaker so I can't give the best answer. This is the page I've used most in my studies
"It doesn't matter what," in my opinion, would best be translated as "Ez mindegy." Mindegy is like the German "egal."
You should also keep in mind the distinction Hungarian makes between some/any could be and probably is different than how English speakers make the distinction. So when you want to say "anything," even though "valami" is "something," it could be the better option.
I think this sentence is quite misleading. It doesn't mean "I'm ready, no need to wait for anything, let's go", not at all. "Én nem várok semmire" could fit that meaning, "Én nem várok semmit" is actually a rather depressive sentence, as if you said "I don't expect anything (to happen)".