It's a little frustrating actually, that when I start up an online match, my mind kind of freezes and suddenly don't know what to do to attack the opponent. I always end up doing fireballs (sonic boom), some moonsault (even though it's very unsafe) and small combos like mp > lk >hp or hk > mk > hk.
Just keep practicing and sooner or later you'll start figuring out how to incorporate new skills into your battle plan.