The main thing imo is how much experience you have with playing the system or a remotely similar system. If you’re totally new to actively engaging in the hobby, the first thing you should do is probably run a one-shot, and whether it’s original or pre-written should just be based on your comfort with working with someone else’s writing versus your own (though you should definitely at least read a one-shot someone else wrote to see an example of using the mechanics, though you absolutely don’t need to copy their style).

Any further judgements can be made after you run the one-shot. There’s a good chance, especially since you like improv, that you’ll find it much more gratifying to write your own stuff, but maybe not and either way is fine.