Your combo is the combination of attack skills in a quick series of strikes that ignore cast-times. Make sure that the skill you want to use next has finished "cooling down" before attempting to cast it.
Use the skill before the bar reaches the end, and only after the first line on the bar.
Always aim for an "Excellent" if possible.
As a "heads-up", you cannot win a PvP without a combo. It's strongly adivsed to practice combo'ing on mobs before you attempt to engage in a PvP.
Each class has their advantages and disadvantages, it is up to you how you mask them and tame the wild strength residing in each.