The first thing you will need to do is choose the side you will fight for, or if a civilian impression might be what you want to do. It is highly recommended that you find the unit you wish to join before buying anything! Most units will have a person in charge, or at least a list, to instruct new recruits on what to purchase and where to purchase it. If you go out and buy a lot of equipment that is not right for the particular unit you will be representing, you will have wasted a lot of money. Hopefully, you will want to be as authentic as possible and wearing a uniform or carrying equipment that is incorrect will ruin your impression. Reenacting is a return to those days when people actually thought about what other people's perceptions of them were. It would, therefore, behoove the reenactor to study how our ancestors handled human interaction and seek to emulate them. Interestingly enough, modern-day folks appreciate it too.

Cursing is definitely frowned upon. It is not necessary to do so in order to get your point across. Substitute period exclamations for curses, if necessary.
Public intoxication indicates that the person has little or no self-control. Imbibe with restraint, and stop well before your wits become totally befuddled.
Wear clothing appropriately. Going around half-dressed, even if you're overly warm, isn't indicated.

Camp Life
Take your fair share of camp jobs. Offer to help set up, bring wood, water, straw, &c. In fact, more than your fair share will be appreciated, and will serve as an example to others.
Keep your area clean and neat and farby (Modern) free. If you see something farby pick it up or cover it up.
If there's a dance, spruce up before you go. The ladies will appreciate it a lot. Remember that dancing of that period was more sedate, and do try not to step on the lady's gown. Dance with as many ladies as are willing.
After a hard day's reenacting, and the supper is over, it's a nice time to sit around the fire with your pards and engage in fun and talk. Around 11 PM or so, however, it's time to turn in and let everyone get a good night's sleep. Reveille is usually early at most events, and many reenactors are not so young, anymore.
After the final battle and you're packed up, take a few moments to make sure your tent site is cleared of all rubbish and offer to assist others if needed.
Before the Battle
Come to the fight with enough cartridges, caps, water, and food. You may be out there for a time. The middle of the battle is no time to scrounge from your pards. Review in your mind the common commands and how you should react.

Be quiet and listen to your own officers and NCO's. If they cannot hear they cannot guide you. If you cannot hear, you can't be guided. Unless you're at REST, silence is the way to go. "Parade Rest" means be silent.
Remember the safety rules.
If you're not too knowledgeable on firing your musket, ask to be in the front rank.
Safety is the primary consideration. Stay calm and follow orders.
Elevate your musket's aim above the heads of the opponents. They will appreciate it. If they're consistently aiming at you, notify your NCO.
If you've brought along your camera, try to keep it as hidden as possible. You don't want to ruin someone else's 'magic moment' with modern things. Turn off the flash, or put black tape over it, so the spectators won't know you're photographing.

After the Battle
You're tired. Your unit may be scattered. Look for your flag and rally to it. There may be a parade for the spectators. Don't skulk off. Do your best to look like a soldier.

At the Sutlers
When confronted with obviously 'farby' gear at the sutler's emporium, it is best to keep silent, unless one of your pards appears ready to purchase it. If you know of a better quality item somewhere else, it is only polite to discuss this outside.

Dealing with Spectators
Questions from spectators: Remember the time period you are when you answer questions.
You are the expert, but if you don't know, say so, and find someone who does. You obviously don't want to get deep into an answer with young children.
You also don't want to get into an argument with a stranger. You are right then, the ambassador of the reenacting hobby and your unit or club to that person.
Above all, don't get bogged down in the causes of the conflict. Suggest that the spectator peruse such in their local library or book store. If you know of good books or magazines, tell them. You will get people still fighting the war who want to argue with you about it. Politely tell them that your duty requires you to be elsewhere right now and depart. Particularly avoid conversations regarding race or previous condition of servitude.