I just bought this really nice Arce Palmatum tree, is about 30cm tall. I want to make it a bonsai, which would be my first. I've seen loads of YouTube videos and tutorials but I just can't imagine the best way to prune it. I understand this very first step is very important as for how the tree will look in the future. I'd appreciate any ideas and suggestions.
I'm fairly new to this game, and am more comfortable with starting from smaller stock, but I'll take a swing at it.
Since your end goal is a 25 - 30 cm tree, and it's already there, it will need a significant reduction. As far as I can tell with this tree species, you're in the sweet spot for pruning.
Personally, I'd prioritize getting it established in it's new pot. Be sure to reduce the root ball by about 40%. If you're feeling really froggy and can pay close attention to it, you can prune it back at the same time.
The lower trunk looks fairly well established, so you won't be able to train in, but you can tilt the root ball 45 degrees (or whatever please you) in the direction you desire. Also take back the main 'trunk' by about half and start training the younger branches. Prune back in the middle to reduce crossed branches and you're on your way. The tree can probably take a loss of about 40% of its leaves. Be sure to fertilize a touch and water it well after that trauma.
Honestly, it's a hard sell to make a mini bonsai out of. You more easily push a medium (40 - 60 cm) with how much it's grown.
If I've given bad advice, please correct me! I'd hate to kill a tree by proxy.