Whilst I have never played a Druid, typically what happens is that you are given a certain rank in a skill when you first buy it. For example, as a Paladin you get Holy Light Rank 1 at level 1. That heals 39-47 damage for 35 mana.
As you go up in levels you get the ability to train (Through trainers) more advanced ranks. This is determined by your level. Holy Light Rank 2 becomes available at level 6 and then heals 76-90 for 60 mana.
Rank 3 becomes available at level 14 and so on and so forth up to a maximum of rank 11 which becomes trainable at level 70.
You do however get skill books at later stages which circumvent the trainers. I've not encountered one yet (Apart from buying and reselling them for profit

) so I don't know how they function in-game, apologies.