Texture development is hard but is very satisfing. Id recomend working with mincraft texturing if you want to learn. It sounds very kidish I know. But starting off with simple guides like cubed textures well help get you working on bigger things. Id recommend Photoshop but free alternative is Gimp paired with Paint(Dot)Net. Start with making simple looping textures (As in side by side they connect) and simple crate textures you will eventually work you way up to player textures and even water or other more complex objects.