Which format is best for beginners?

Title. I like to play on showdown here and there, but the only team I've really made is a subpar UU team. Should I pick another format? Or is any a good place to start?

Generally speaking, I recommend OU as your starting place just because it's the "standard" meta game with the biggest playerbase. Honestly though, you can focus on any tier you like it just means learning a different meta. If you want to keep playing UU, then you should familiarise yourself with common UU threats and learn how to check and counter them. This thread is your best friend for learning the UU meta. These might help you out with learning some of the more general aspects of competitive play, and there's loads of other guides just a quick Google away. I'd also recommend starting out with a sample team made by someone with more experience (there's a lot of these easily found by Googling too) while you get a feel for things.

If you ever need any more help, feel free to drop me a message. Fair warning though, I'm a bit rusty atm and UU was never my preferred tier.