He's right over/under is the best option not to many people utilize it from the back. Most people use the double underhooks. The grip is important to like gable grip or even a S grip.
I think it's more "natural" feeling for most people to use the double under grip. If you have excellent hooks then it can be a somewhat effective grip if done properly. What I like to do with the double under is grab both of my opponents wrists (same side). Once I get that grip I have a pretty nice grip on the armbar if they roll out either direction. It has it's place, but the over/under is much better to control them and keep them locked into the position. Being that you are knew to BJJ I'd focus first on being able to establish and hold the position...once you can hold your partner in the position start working for some subs.
Also if you have the over/under and you feel as though you are losing the position switch it up to the wrist control grips and wait for the armbar. Just remember that if you are laying on your back or your side the armbar is very tough, it's best to keep them sitting up.
Another trick that Tim Credeur showed me that he does a lot is when people lay back on you and put all their weight on you. Go on the under side and secure a half Nelson. Once you have the Nelson kick out the opposite side foot and roll into mount while snaking the Nelson around the neck...you end up in mount with a nice guillotine choke.
Hope that helps some.