So combos work in two parts.
First, combo fields. All of your abilities will say whether they give a combo field and, if so, what type. You can tell a combo field is up because there will be a white line around the field (often a circle or long rectangle.)
Secondly, you have combo finishers which are also typed (combo finisher: blast, combo finisher: whorl, etc.)
Using a combo finisher through a combo field will apply some benefit. So whenever you see a white bordered area use your combo finishers. Similarly, try to throw down combo fields where they will hit a bunch of players.
For example, look at the Mesmer Staff. Chaos Storm creates Combo Field: Ethereal. Phase Retreat is a Combo Finisher: Leap.
You can plop down a Chaos Storm, go inside the area, and then use Phase Retreat. Because you used a combo finisher through a combo field you get the combo effect – in this case you are given free Chaos Armor.