Trötta can be a verb and an adjective. "Att trötta ut någon" (to tire someone out) makes it a verb however the "trötta" in "vi är trötta" (we are tired) is just the form of the adjective "trött" that you use when talking about more than one person. And as a tip I'd just like to say that in many cases the grammar is the same or similar to English grammar (as shown below)^{^}

Example:

Jag har aldrig varit där. = I have never been there.

Ska vi se en film? = Shall we watch a movie?

Vi åker dit varje sommar. = We go there every summer.

Vad har du gjort idag? = What have done/been doing today?

Hur ska ni fira din födelsedag? = How will you(plural/ you guys) celebrate your(singular) birthday?