Learn Finnish: Selvä! Minä olen valmis.

Yes! The word selvä literally means "clear". In everyday Finnish, it is very commonly used as an exclamation meaning "okay", "alright", or "understood", similar to saying "crystal clear" in English.

Could I just say Olen valmis instead of Minä olen valmis?

Absolutely. Because the verb ending in olen already tells us the subject is "I", the pronoun minä is optional. In spoken Finnish, it is very common to drop it and just say Olen valmis.

Can valmis mean anything else besides "ready"?

Yes, valmis can mean both "ready" and "finished" or "done", depending on the context. Here, right before starting an activity, it means "ready". If you said it after completing a task or eating a meal, it would mean you are finished.