The rotation you use is dependent on which skill tree you are invested in. For example, you should only be using rupture if you are annihilation specced. I will try to give what advice I can:
Any spec: Start most combats with force charge. If you are consistently getting knocked back, however, try running into combat, and then using force charge after the first knockback to surprise them. Use cloak of pain often. Its cooldown time is pretty short, and it lasts as long as you are taking damage. Marauders are very squishy without their defensive cooldowns. Assault, Battering Assault, Vicious Slash, and Ravage are useful attacks regardless of spec. Deadly throw should usually only be used if you need to attack from mid range. If you are high enough level for force grip, it is also useful for keeping you alive and attacking.
Carnage spec: Add force scream to your rotation, especially when you get the ataru form buff for it.
Annihilation spec: Use rupture every chance you get, to keep the dot up. Force scream is not very useful, unless you need to attack from outside melee range for some reason.
Also, you should check to see if your gear is close to your level. If not, there are many ways to get better gear (level 20 pvp gear on dromund kaas, planetary comm vendors, and flashpoints.)
I hope my advice is helpful.