If your looking for custom textures you could use - search up a ‘texture pack’ in the toolbox If you find textures that you like you always try to make them work or if your wanting to go for a better approach you could attempt to create your own based off off other textures created by other developers.
To create your own textures you could use different programs or software to make your own, such as gimp, paint.net, photoshop ect. For creating and approaching to make your own materials The dev forum has resources - tutorials for either making your own or using free texture here is something you could take a look at: texture sites