Get Gimp - it's free Go through the tutorials - lots online, learn how each tool and option in the menus works. Even once you get proficient go back through everything looking for quirky ways that others use it to get the effect that they need.
Try paintnet: https://www.dotpdn.com/downloads/pdn.html Not much to learn there, just download it and start using.
You can also use Photoscape, simple and easy to use. If you need more features and functions, so, go with Adobe Photoshop
Why not think about YouTube? I think this is the best way people love to train their self without cost. Now I'm going to serve you the name known by Lynda. On your beginning days, you can train under lynda.com but it is a bit costly for the membership registration. So, check something special there in Adobe Know How. Here the other resource for photo editing that uses Photoshop. http://www.good-tutorials.com/users/shumicpi