Before you decide whether to be a shaman or not, consider its possible focus. There are three to choose from, the first one being the Path of Gork. This route allows the shaman to hone his combat skills, focusing on building up enough Waaagh! power to quickly and decisively kill his enemies. If you love dealing damage, this is the right path for you. As an added bonus, the more you kill, the more you can heal using the built up Waaagh! power. This path simply emphasizes killing enemies first, and then healing allies.
The Path of Mork instead focuses on healing allies first. The best part of this path is that it allows the shaman to build up Waaagh! power while healing, which helps him then destroy enemies quickly after healing his allies. So many people dismiss this path because it focuses on healing, forgetting that the more healing a shaman does, the more damage it can deal.
The Path of Da Green works on the assumption that anyone who is not green--of the Greenskins Army--must die. It focuses mainly on debuffs for enemies, and buffs for allies. This is the path to take if you want to be unique, and can't choose between Mork and Gork.