Originally Posted by Me
Most everything has already been covered by DJ and BG, but I would like to point one last thing out:
There's a very important materia to have for use in learning enemy skills, and that's the Manipulate materia. Some Enemy Skills (such as White Wind or Mighty Guard, for some examples) will never be willfully cast on your party by the enemy, as they're supportive/healing, rather than damaging. The Manipulate materia allows you to manipulate an enemy (who'da thunk) and enables you to have access to some or all of that enemy's "attacks" (which you can in turn cast on your party, or the character with the Enemy Skill materia equipped).
Manipulating enemies is literally the only way to learn some valuable Enemy Skills. I believe Cait Sith may come with one equipped (actually, I think that's Transform). Whatever the case, get one.