So what should I train in?
What do you want to train in?
Just about any facet of CB can be tweaked to perform well. It all depends on taste. Do you like Ranged attacks? Do you prefer Spellcasters over Fighters? Do you want to run a single minion, or a full team of four?
If you're not really sure, or not fussed, then there are a couple of suggestions for new player strategies. From Dispel Magic with Fireball, to a 4 minion RoS AS/GA team, Junctioned Familiar, or Single Archer.
The 'Help!' wiki also lists some explainations and ideas on these.