If you're buying it... Buy a dime bag first - you won't need much with a low tolerance, and it lets you see what the person who sold it to you has to offer. Plus, it prevents you from getting cheated too badly just because you're a newb.
Seeds/stems=bad. Avoid these. You might get some, and if you do, thats life.
Do NOT try to roll it if you never have before. You will almost certainly fuck it up without some instruction.
Smoke a little bit, then more if you want. Don't keep toking just because whoever you're with is - this leads to you getting ripped off your fucking ass, and you will not be a happy camper. This is when being high can go VERY, VERY badly. If this does happen, go lay down, and listen to some calming music. Helps a lot.
If you don't get high the first, or even first few times you smoke, don't freak out - it happens. Took me like 6 times lol.
Also very important - don't be afraid to ask more experienced people a few questions. They will more than likely be happy to answer, and you can learn something in the process.
And... I highly recommend you listen to some music. Make sure you bring your ipod and have some speakers or headphones. Will make you a happy camper fo sho.
Have fun!