Aww, thank you so much!
PICO-8 is pretty friendly to learn but that might be a bit of a jarring leap directly from Game Builder Garage, especially since it would have a very different workflow as far as I can tell. Since Scratch is free and easy to access I figure you might as well try tinkering with it. Around your age I believe I was mostly using GameMaker, which was a great way to learn since it had both drag-and-drop and written coding options.
If you're really interested in PICO-8 specifically, I've heard good things about the LazyDevsAcademy tutorials. You could maybe look at those to get more of a sense of what it's like to work with. On the PICO-8 forums (https://www.lexaloffle.com/bbs/?cat=7#sub=2) you can also view the full code of any cart by clicking the little "Code" button on the lower edge of the player window if you want to look at how folks did things.