If you tend to be cheerful and positive no matter what's going on, your friends probably describe you as upbeat. When your soccer coach wants to be encouraging, she gives the team an upbeat speech.
Need to cheer yourself up? Spend some time with an upbeat friend, watch an upbeat movie, or listen to some upbeat music. This informal adjective comes from the word's original musical meaning, "the beat of a bar at which the conductor's baton is in a raised position." Music writers in the 1940s used the slang phrase "on the upbeat" to mean "getting better." The "cheerful" meaning followed around 1947.