Happiness is where you have the pokemon on your team, you heal it when it get hurts, you feed it stat raising items like zinc, etc, you can feed it the coloange scents, but most of all don't make it faint!
to evolve a Golbat in XD is the same way to evolve a Golbat in the other games. You keep it in first, you battle a lot with it, bring it to the pokemon centre when it needs it, etc.