Before you select your first soul make sure you look into its talent tree to make sure its something you want to play. When you select your 2nd and 3rd soul make sure the compliment your first. If you pick a bad combination or spec wrong don't worry its very easy to get other souls and respec.
Don't forget to change the UI around till you get it just right.
Building on PsyGunWolf, pick up at least one of the other two souls that compliment your main one at the beginning... at least on your first character. It'll help getting to level 13 when you can begin picking up more souls and at that point you can pretty much explore different builds as you please until you find the combination that works for you.
Proud Rancher of the Baku. RIFT: Aldin - Justicar / Sentinel | Server: Faeblight