Other than practice I would stress the virtues of conditioning above anything else in submission wrestling.
I'm a strong guy but Ive always found myself struggling the most against fighters with more stamina, even fighters significantly lighter or not as strong as I am.
Persistance is another thing youre going to need because if youre new to the sport youre going to have to get comfortable with the fact that youre likely to get thrown around pretty bad for a couple of months before you reach a level of ability where you can first of all resist an opponant and then eventually give him a challenge. I used to come home black and blue and wondering why I was putting myself through such pain when I first started.
Finially I would say you need to develop mental toughness. Theres all sorts of nasty tricks in submission wrestling that isnt going to submit you but is damn painful and uncomfortable and when some guys cranking your neck, digging his elbows into your thighs to break your guard, has his forearm accross your throat or the top of his head jammed into your face it can be really tempting to give up - the trick is not to and also dont let your opponant know it hurts so no grimmacing, moaning etc.
Everything else you will learn in time