Here is the breakdown: Get some 3D modeling software (recommend Blender because it's free) learn to use it, make simple models for objects or scenes, paint over them in Photoshop, and stay away from pressure for opacity, that shit is heresy and makes new artists' digital paintings look like mud. learn first to paint with solid, no pressure sense brushes and only use smooth, low opacity brushes extremely sparingly.
I recommend you to check out space art tutorials for photoshop, and as many as possible, though space art might not be your thing, the various tutorials will tend to show you different tools in photoshop you might not have known before, you'll probably not use them all, but it'll be helpful to know they exist for the few cases they are needed.
I also recommend you to find tools to help you where you are weaker, for example: I hate doing prespective work, so I use Blender to compensate. If I want smooth lines, I'll open lazy nezumi to smooth them out. Never get stuck with the "But it's cheating" mentality, we aren't living in the times one had to gather ingredients to make paint, you are using a PC, as long as it helps making your image better, everything is fair game (Unless of course stealing someone else's work).