Golbat evolves by friendship into Crobat, so you have to get its happiness up high enough for it to evolve and then level it up. Their happiness increases when they level up, when you give them items and haircuts. You can check their happiness by talking to a woman in Goldenrod City just above the bike shop. When she says "It looks like it really trusts you" it'll be almost ready or ready to evolve.
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Golbat will evolve if it has max friendship with its trainer which means it can evolve at any level.
Make a Golbat level up on a high friendship level.
golbat can evolve at any level if its friendship is up to max power...or...it can evolve if you give it a berry that gets the Pokemon friendlier then get it one level higher and ... evolution
You need to level Zubat up to level 22. Then it will evolve into Golbat.
Nothing. You need to make Golbat to have Friendship or Happiness.