The 2 websitestextures.com - find thousands of textures here for free!
imgonline.com.ua - a ukrainian website that I recently found, but has english support. The link here links to the english language. Has a lot of tools, like seamless texture maker, free upscaler (it's called resize for some reason) and others! Although, some don't work best, it's still free. Can't believe I didn't find it earlier.
Photo editingpaint.net and its addons/plugins - an extremely popular choice, and for a good reason. It's free, it's easy to use, and it supports addons. It is basically photoshop but free. Getting plugin packs? Here are some I can recommend for starters.
Plugin packsBoltBait's pack - An essential. Get it. Has shapes and a lot of plugins in it. You might want to get it because of Creative Text Pro; with it, if you select to apply the material on objects, you get materials! Stuff like copper, rust, diamond plate, etc.
MadJik's pack - Also an essential. It's more focused on photo editing stuff, but that's what you need! Distort, blur, color editing, very useful stuff.
TechnoRobbo's pack - Might have to install this one manually, but it is 1000% worth it! It has a displacement map, which is very useful for applying realistic text on bumpy stuff, tiled pixels for tiled glass and mirrors, and others! Most useful pack here.
It was a very short tutorial, but the stuff here will save you days and even weeks of suffering in trying to make a good texture. Thanks, and happy texturing!