One thing you can obviously do is go nursery hunting and seek out junipers to practice on that have trunks with nice movement and taper. There is lots of bonsai terminology and lots of variable advice you'll hear in this forum, which is what makes it great. You've got some good advice already. Be careful collecting now, but most importantly, whatever trees you do work on, learn the species, and plan to develop patience.
The worst mistake I made early on is chopping a young tree into the bonsai I'd hope it to be years later. Find some good YouTube channels, learn, dive deep into the forum because your questions have likely been answered (not all), and most of all, have fun. You'll get frustrated, maybe lose the drive, but just keep at it.