What should I know before my first MTG draft?
Make sure you know the policy on rares - will rares be re-drafted at the end? Do you keep all rares you draft? Is it a 'winner-chooses' system? Most of the time you'll keep what you draft, but not always.
Talking about the cards you're drafting is generally frowned upon - reading signals and predicting what your opponents are drafting is a big part of the strategy, so discussing your picks is disruptive.
The above applies for discussing what you think your neighbors are drafting, too.
Find out if basic lands will be drafted or removed from the pack before drafting.
Find out what the policy is with regards to looking at your cards during the draft. Some groups won't allow you to look at your picks during drafting; some allow a minute between packs to review
It's really not acceptable to complain about your luck, bad as it may be. Part of drafting is making the best of what you have, and half the skill is mitigating any poor luck with good decisions.
There will almost always be a 'pile-up' where someone isn't 100% sure of all the cards, so needs to read them before choosing - be patient with them! However on the other hand, don't take needlessly excessive time deliberating over a choice.
Make sure you are aware of the drafting order - usually it'll be first pack left, second pack right, third pack left, but some pods do it differently. Passing the wrong way can really confuse matters.
If you find a couple of packs waiting for you, make sure you go through them in the right order. If you're not sure, check with your neighbor. 
Take your own lands. Some stores will provide lands but occasionally they'll run out, and having your own can take the pressure off.
Sleeve your cards. All kinds of good reasons to do this; preventing differences between lands and the slippery new cards, protecting cards (especially if rares will be redrafted).
Don't be afraid to ask a judge. It's always best to be safe than sorry.
Don't be too shy to ask spectators to back off if they are distracting you or disturbing your game
It's usually considered polite to offer your deck to your opponent to cut or shuffle to their satisfaction before playing. Not doing so will cause suspicion that you may be deck-stacking or cheating in some other way. In more formal, higher level tournaments it is mandatory to present your deck.
Bring a pen and paper for life tracking and counters to use for all kinds of stuff (+1/-1, etc). Dice are considered too accident-prone in competitive play. If you're just in a casual store draft, dice should be fine. Phone apps are also considered ok for life tracking. In more formal, higher level tournaments it is also standard to track your opponent's life total.
Sometimes you will be unfortunate enough to come across someone who uses cheap tactics to try and put off their opponents. For example, arranging their lands in front of their permanents, or tracking their life in base 6. Do your best not to let these maneuvers distract or fluster you. If you have any concerns at all, call a judge - it's what they are there for. (and if someone's life tracking is obscured in some way, keep track of it yourself, carefully, as they may be using these odd counting methods to disguise cheating.)