first of all : you picked a great class for a beginner! not too many buttons or fancy mechanic that distracts you so that you are able to focus on the play and positioning.
I will focus in my description on Groupplay and or Scenarios.
1) your default guard is on a DPS - this is the Target you have to protect and support!
this point is tricky especially in random SC - you need to know and learn who is a good dps, because a bad dps Player will "kill you" with his playstyle while a good one can win you the sc.
2) you support this dps with everything you can!
as a KoTBS this means GUARD, KNOCKDOWN, Punts concentrate on one in the beginning (Guard) and add the other 2 if you are fine following and staying with your target.
2.1) we stay with the assumption you guard a melee dps and you can guard and follow him
now you want to support him further, make sure his target is snared all the time - use your Knockdown on his targets if they either close to escape or have lost some life and are between 70%-30% health. so he can finish off the target.
2.2) ok now you know how to guard/snare and knockdown a target
The next step is to Punt(banish darkness is must have) ! why ? because the tank on the other side is tryin to do all the same things you try to do. And with a good Punt you remove him from the game! you need to see who is guarding the target of your melee dps and try to punt him away so he is out of guard/support range.
2.3) in case you supporting a ranged dps
step 2 is only used if he is in danger, otherwise the points 2.1 and 2.2 are more important. you now push in the line, assisting him knocking down his target, and in case of a ranged dps you dont punt the enemy tank, but the target of your ranged dps in the direction of him (best if timed with his burst skills).
3) switching Guard
is the skill to see where the enemy assist train is moving and guarding the target they pick instead of staying with your dps. This is tricky again, it can be benefecial to do this but you can also expose your before guarded dps if he is not aware of this.
if you either focus to master points 2.1-2.3 or 3 first is up to your preferences.
sidenotes
to fill your role several things will help you a lot to do this:
1) tactics: banish Darkness (better punt) slice through (AoE snare)
2) the right addon to see the most important information you need to be aware of:
- enemy immunity Timers for Knockdown and Punt/root
- the Guard of your enemy target(to faster decide who you have to punt)
- if you assisting a BW in section 2.3 it helps alot to see his Boiling blood and the timer, because all his burst is centered around this spell if you can manage to punt the target while the BB is goin off its is most likely dead

3) due to the importance of punt use your AoE root only if it is absolutely necessary ... it gives immunity to punt ...