When you only get 1mpg, you need a gas tank bigger than your car. When I started BJJ, gross inefficiency was my biggest problem. It will be yours, too. You won't know what to do, everything will feel foreign, and you will learn some bad lessons at the start perhaps, like thinking you can bench press your opponent off you. This works for about 1-2 minutes, and then after that you will be a grappling dummy.
I personally wouldn't bother with cardio. Grapple, and the cardio will come. Laying on your back at the end of class for 10 minutes, willing yourself not to vomit is a rite of passage. Whatever you do, try not to gas and sit out the rolling. You will need to learn to not try so hard. Grappling is counterintuitive. The sooner you learn that your innate grappling reflexes are worthless, the sooner you will improve. If you have the cardio to spaz for 5 minutes straight, you might even beat some people but it will teach you bad lessons.
Your expectation of your first month or so should be that you will be lying under sweaty dudes for what seems like an eternity, struggling to breathe under side control, as they smother you and then move your limbs to where you don't want them to go. Don't think you are going to be tapping people out. Your absolute last priority should be learning submissions.
If you REALLY want to do some prep, I'd buy the Saulo Ribeiro (Jiu Jitsu University) book and study it at home. Especially study the survival and escape section. You can practice it yourself. This will give you something useful to try when someone is on top of you smothering you, instead of the WB bench press escape of epic fail.
Also ingrain "Position before Submission". Accept that you will learn the hierarchy of positions, and how to ascend this hierarchy before you worry about submissions. The basic positions are back control, mount, side control, knee-ride, closed guard. Google them. As you don't want to be smelling sweaty gi for long, I'd advise learning an escape from back control, mount, side control, knee ride, and a sweep (e.g. basic sweep) from closed guard. This will make things more enjoyable fast.
If you get the opportunity to roll with a purple or above who you outweigh by 30lbs or so, do it. Getting schooled by someone that small will teach you on a visceral level that technique is superior to muscle. You can go out and learn powerlifting and probably be stronger for 30 seconds than anyone in the gym, but it won't help you beat a guy with lots of technique.
Learn from everyone. 3 stripe whitebelts on up will have stuff they can teach you, and will love to teach you. Ask the good people how they became good. And write down bits you recall (technique, opponents, where you are getting caught, your successes and what you want to work on) at the end of every grappling session.
Good luck!