If you never played RG before be warned.
CS and OB are both pretty hard to master due to their mechanics + general position bugs.
CS is only good, if you are properly geared for that purpose + you hit the straight line so you need to mob the monsters on a straight line to hit them. Damage is mehh... without much STR/+STR Gear and Ranged % Gears, so you should go with Overbrand first and then master Moonslasher or Cannon Spear.
Moon Slasher and Earth Drive are the easiest to use due to 360° hits, Cast Time + Delay are the problems + ED is like the name says, forced Earth Element, but you could use it to level in Magma 2 and in Rachel sanct (ive done it before the Giant Shield was released with the Imperial Guard, works well) if you cant 1 Shot em, Magnum break or Moon Slasher as finisher. MS is pretty weak thou + it is affected by your Overbrand level.
So the only viable AOE as Newbie Royal Guard is Overbrand.