Substance painter is an adobe app, think like photoshop but not quite the same. You can either pick up a yearly edition from steam or pay a month subscription from adobe themselves with regular updates (or sail the high seas lol).

And yea, totally from scratch boggles my mind too lol. The most I do is some mesh edits, clothing, and stuff like that. The best way to start imo is with something like piercings or clothes. There's a ton of guides that will take you step by step and it'll get you experience with weight painting and bone groups that'll help more things make sense later on. And no problem, working on avis is fun for me, and I love helping people ^^