MX Simulator

Need some recommendations about what software I should use for making skins. Anything helps

Adobe Photoshop and Illustrator are going to be the best tools, and will be most compatible with everybody's files since they're industry standard. If you're looking for something free, GIMP is a common Photoshop alternative that's feature rich and has existed forever. I've heard great things about Clip Studio Paint too - I've never used it but it's popular for digital painting and sequential art.

photopea.com could be an option too if you can't afford photoshop. It's pretty much a clone of photoshop. It doesn't have all tools and some tools like content aware fill aren't quite as good as photoshop, but it's definitely enough to get the job done for skinning.

Krita is another good freeware paint program.