I'm fired up about my first day back on set.
I found this sentence in a cartoon movie. I don't understand the second part of it. Good doesn't help.
Here's the meaning of the expression fired up as defined by the Macmillan Dictionary:
feeling very excited about something or impatient to do it
While I couldn't find any reference online for the adverbial on set (see set construction if you want to know what the word set actually means), my knowledge of the English language tells me that it should be something relating to the rehearsing of a play or the making of a film (see the adjective on-set). In other words, something that's happening on set is happening during the time when a movie or a TV show is being made.
So, my first day back on set then means that I've been away from work for some time and now I'm back making whatever movie or TV show that I was in the process of making (probably as an actor or actress) and it's my first day after this long vacation.
Here's a picture of a movie set (or a film set if you're in the UK) if you're still wondering what it actually is: