While leveling I would just pick one and put all my points in it. Once you hit 50 you can repec to a more hybrid spec. It just makes leveling easier, but some of the skills you will not use in groups once you have your full rotation down.
First I think you better understand what DoT are if you are playing the Vigilance tree. DoT is damage over time.
Stacks are a buff or debuff that build up on you character or on the character you are attacking. They will tell you which attack should take Priority and which attack you need to use to build up stacks for other attacks. It looks like two of the stacks you will be looking for are Sunder Armor (debuff) and Force Rush. There may be others so read and turn the pages too as it shows you other useful information too. Like Stat Priority, Item Modification and Builds.