I have learned that with proper care and feeding "newbies" can become productive jiu jitsuka and a credit to your academy. I've been trying to write down in one place all the things I wish all newbies had to hear/understand before they were allowed to roll. Here is my list so far (please comment with any of your own - or if you think I'm way off)
Respect your Partner
Always respect your partner. Seriously - RESPECT your partner. You are responsible for his safety as well as your own (and vice versa). Don't jump all over them with flailing knees and elbows. If you think you have an arm or a leg don't just twist/spin/yank at high speed - if you "have it" you should be able to manipulate it smoothly and under control. No striking, slamming, biting, poking, or grabbing/twisting small body parts (fingers, toes, ears, nose)
Know How to Tap
You need to "tap" multiple times and firmly enough so that it is clearly felt by your partner. If you can't tap on their body, tap loudly on the mat, if your hands aren't free use your leg, and finally don't forget verbally - just yell "tap, tap, tap!"
Know When to Tap
There are consequences for not tapping - a choke will put you to sleep, an armbar will put you out for a few weeks, a kimura will send you to surgery and a long recovery, and we won't even go into leglocks - so just tap already. When you first start rolling you may not know the difference between something that is just "highly uncomfortable" and something that is going to mess you up - if you are not sure TAP - you can ask the instructor or an upper belt about where you are in real danger. It won't take long for you to know the difference yourself - but until you do be safe. Special Note: In class we are training. We are all working together to get better. Gym taps do not count - no extra brownie points for getting one, no frownie face on your permanent record for giving one up.
Respect the Tap
When you have a submission applied it is your partner's job to tap, but it is your job to notice and immediately respect the tap. Release immediately and be careful in how you "unwind" from each other. Also be aware of your opponent's position, he may not be able to use his hands on you and could signal verbally or by tapping the mat with his hands or feet instead. If you think your partner is tapping but you are not sure, let go - it is better to be safe than sorry. When you have/are working for a sub you should be alert for your partner's tap.
Other Reasons to Tap in Class
If you wind up with body parts unintentionally tangled up in the gi it is okay to tap and reset (you will learn about gi manipulation and the correct way to grip) - that said, don't stick your fingers and toes into their gi unless you want to lose them. Also, you will need to learn how to breathe when in uncomfortable positions, such as someone laying on top of your face, that said I would rather you tap and reset rather than gag and throw up on the mats.
Tapping an Upper Belt
If you tap an upper belt he/she LET you do it (99% probability) so that you could learn - don't brag about it. It does not score you extra brownie points or get you promoted faster ("No, you don't get my belt if you tap me. Calm. Down." ~ Megan at Tangled Triangle). Most upper belts enjoy helping you learn and will often let you work things through on them, but that can end very quickly (and unpleasantly) if you let your ego get in the way of your common sense.
Don't spazz. Seriously - don't spazz - it makes you dangerous to yourself and to your partner. Don't explode randomly all over the mat for 30 seconds and then be ready to puke. Don't freak out in bad positions or when you're caught in a sub, it's just training.
There is a fine line between good pressure and being a jerk - until you have been rolling for a while you don't know where that line is so you might want to ease up.
Don't Muscle It
Don't bully your partner. Try and relax and not be stiff all the time - there is a difference between a good anatomically strong "frame" and muscling a bad position. You will learn when you need to go fast, when you need to apply pressure, and when you need a little extra umph. You don't have to go soft and floppy but lighten up a little and you will roll better.
Control Your Submissions
Finish a sub under control (and slow enough for your partner to tap) - if you can't do this you didn't really have the sub. You can grab it/sink it in quickly but always apply the finish slowly and under control.
And Finally Some General Housekeeping
Wash your gi , Wash your gi, Wash your gi (and your belt too) after every training session. Nobody wants to roll with a festering pile of stink and biohazard.
Trim AND file your nails. You will get enough scrapes and scratches that can't be avoided. If you don't want to do it for me, do it for you - long finger/toenails are just waiting to get ripped from your digits.
No jewelry (this includes all piercings/plugs) - unless you want whatever bodypart the jewelry is attached to to be separated from the rest of your body - leave the jewelry at home. This also goes for those "special" piercings that I really don't want to know about when they become embedded in ways they were never intended.
This may seem like a lot of stuff but I've found it can be covered in about 10 minutes - slightly longer if you want to have examples and demonstrations.