The surface itself depends on what I wanna make. If I'm just making a random planet, I just generate a random texture playing with noise and colorizer gradients. If it's a major world that I bring up multiple times, I have maps saved that I made in Photoshop.
The clouds are a separate texture. They are nothing more than a cloud map (royalty-free easily found on Google) alpha with bump applied. The atmosphere is made with a special plugin