At level 10 you will be given a chain of missions from your hunter trainer to obtain a pet, complete this chain of missions and you will be able to tame pets from the wild.
You can have 1 pet active at any time but you can store others by using your local StableMaster. Up to 5 pets can be stored but the costs increase the more pets you need to store.
To change pets you either abandon the original pet and tame a new one in the wild or Stable your original pet and tame a new one.
Edited, Jan 1st 2009 7:21pm by EbanySalamonderiel