Being on top isn't a problem! But if you do wind up on bottom, guard is the only place to be:
Don't lay flat on your back, keep your hips active;
Play offense and don't let him settle in; use your core to crunch him forward and to the sides; keep pressure on his head and don't let him posture or stand up-- keep tying up his arms deep in your armpits, do arm drags to threaten taking his back, keep a high guard and threaten arm bars and triangle chokes by isolating his arms away from his body.....good luck.
I could not have said that better myself, honestly!
Rep is coming to you sir.
Only other thing I would add is that be aggressive doesn't mean be careless. I know a lot of guys who try to play the "active guard" and fail hard at it because they just throw things up and don't really think about the consequences. Playing guard you have to be 3 steps ahead at all times. It will come easier with practice, like everything in BJJ, but it pays to be both aggressive and passive. Learn to bait opponents and learn to use your technique, not your strength. You can gas yourself on the bottom much easier trying to force something.