There are multiple ways to achieve this:
- Levelling up. Golbat will like you more the more levels it gains. This, by itself, is rather slow.
- Vitamins. An easy, albeit expensive way to get a Pokemon to like you quickly is to give it a lot of these. Since it's Golbat, 10 Proteins and 10 Carbos will do the trick for him.
- Stat-dropping berries. These berries will make your Pokemon happier, but at the cost of 10 EVs off of a given stat indicated by the berry itself. Best to go for berries that drop Defense or Sp.Attack in Golbat's case.
- There's a girl somewhere (Veilstone I think) who will massage your Pokemon to make it happier. She can only do this once per day, and since it doesn't make a noticeable difference really, it's not as effective as the other methods.