Do you have any tree already? If you don't, consider buying one, a simple, small leafed tree...
Maybe native species (adapted to your enviroment), or a chinese elm for example, something you can practice on...
Big leafed trees are not easy to make bonsai.
And "Yamadori" (tree collection from the ground), demands a bit of practice, so if you want to practice, go ahead, just make sure you are not doing it illegally, you can do it on your yard, or a friend's farm/field, etc.
Where I live, collection of native trees in public lands are illegal, and for a few species, even on private property...