I learned a lot of stuff on YT, the tools I use the most are Quick Selection and Magic Eraser tbh
First you select what you want with Quick Selection, then if whatever you're selecting has hair that you'd like to include in the selection you have to use Refine Edges.
After you finish making the selection, copy it and start a new project and paste your selection. Then resize the image to 100x100 pixels and delete the background so that it's transparent
Here's a quick video I looked up that might help lol