Shadow Woods said:

Hello!

I started out using GIMP to edit/create my manips. It's a free software, though some people do find it hard to understand. Personally I never had that hard of a time grasping it and can try to help you out with that if you need.
I now use PS (Photoshop) from Adobe and really like it. I can be daunting at first because there's a lot in the program for you to use. I learned it in a few classes so that exposed me to a lot of the different tools and workings of PS which helped. Plus GIMP I found similar so that helped my transition. For either I suggest YouTubing a bunch of how-to's and tutorials!

For finding stock to use, Unsplash is a great source. They have high-quality stock (all free to use and anywhere which makes it simple). I also use DA (DeviantArt). I mostly get my horse stock or other random stuff from here, I mostly use Unsplash for my BG (backgrounds) now. With DA though, there are some stock that you have to buy. Also not everything on there is stock. Even if you search for stock make sure that it says so in the title/description or whatever and follow their rules. And look at the crediting requirements for HEE. It's in the art forum pinned near the top.

Hope this helps :)

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Edit: As far as how you do manips, that's a very lengthy discussion and each person does it differently and has there own style. Just give yours a shot and see. But also YouTube tutorials are helpful and sometimes artists will post JoinMe links in chat to let you watch them do art. There are also numerous guides in the art forums I believe, but I am not positive.