Hey Dan, great to hear that you will be starting this class brother!
I started taking bjj at the beginning of this year, so I have been at it for two months (been doing Judo for 2 plus years). Wow is is it different then Judo. I like it though b/c we work more on ground technique, which is less high impact then Judo (mainly throws).
I agree with enditwithaleft: don't be afraid to tap. If you are uncomfortable then tap. When I first started I was not used to leg locks (they are illegal in Judo), so I tapped quick the first time I was in one (even though it probably wasn't in). I will always take a choke longer then a joint lock (getting choked out won't leave you with lasting problems).
Remember that your training partner is there to learn as well. Don't try to break them. Go hard, but when you get that first arm bar, or whatever, don't try to crank their arm off. That said give them hell with your first triangle or any choke.
You said you won't be rolling with black belts, which I think will not be the case if it is like hear in the states. You should want to roll with the black belts. They will teach you a lot and know to show you the proper technique. You would have more of a chance getting hurt by me (a newb) then someone who has lots of experience.
Worry about position for the first little bit. I cam in from Judo thinking I would do ok (which I kinda did), but quickly learned I needed to work on my posture both in guard, half-guard, mount, and everywhere in between. You will get controlled, swept, and put in bad positions, but just learn from each time and you will see how much better you get. My first week (I went hard b/c I have been grappling for two plus years so they threw me to the wolves) I got tapped a bunch by the blue belts, everytime we rolled for 5 minute sessions. I am now down to mostly only getting tapped once or twice in a 5 minute roll with a blue belt (the instructor will always tap you at will, probably).
Watch all the vids and competitions that you can. I like the Marcello Garcia vids. Listen closely to the instructors. Don't get frustrated. You probably won't be good after two classes, but you will get better. Remember that your club is your team, it is not you versus Joe or Dave. You guys are a team, so make each other better. Don't forget to have fun, and get a good workout.
Hope you have a great time!