What sort of textures?
If you want some tiled textures there are good sites and it’s easier to download them than to make something like wood or vegetation from scratch.
It’s easy to change color/details, so often better to try and change a texture to be what you want rather than create from scratch.
Like metal, you can download multiple metal textures - start with something a bit dirty, download a scratched one and overlay them. Lower the saturation a bit, tint them. Take the scratched one, increase contrast to isolate the scratches then tint it and apply it as an overlay… and so on. I used GIMP to do this. With some simple techniques you can take the free textures and make loads of different things.
Alternatively you can get a load of decent brushes for GIMP and create your own textures pretty easily. Anything dirty/grungy is pretty easy.
If you don’t want tiled textures and are texturing a model I usually start with baked AO and flat colors. From there I start applying tiled textures or just work by hand using brushes (loads of cool free ones for GIMP, but it takes a bit of time to get used to it).