You can't get better playing CPU's. You HAVE to fight real people in order to get better because CPU's don't fight anywhere close to humans do. I would say first thing to do is go to training mode and practice spacing your moves against a stationary opponent. Once you feel you have the spacing down, practice spacing with the CPU set to "jump" and "walk". Then after that, just practice warm up type stuff. Just run around basically.
http://www.youtube.com/watch?v=YGcPUyndlu0
This is a video of training exercises for Marth. Come up for something similar for your character. The objective here is to get
command of your character. That is the most important thing for any player with any character is to have command of them. You need to know exactly what you are doing at any given moment and
understand why you are doing it.
You can't get good without great spacing and command of your character. After that, what will determine if you progress or hit a plateau is how smart you play. M2K has amazing spacing, but he wouldn't win if that's the only thing he has going for him.
Remember: Smash = Spacing + Command + Smart Play