I didn't take the time to read all the replies. I have given the "first con" panels many times.
Make sure you have your budget in hand, especially if you are cash tight. There is nothing as embarrassing as trying to live out of the con suite (if they have one) for food. Make sure you have funds to eat on, stay on, and go home on.
Try to maintain your 6/2/1 during the con which is six hours of sleep (minimum), two meals, and one shower.
Try to go with friends or meet friends there if possible. I know many furs who feel left out or alienated because they don't know anyone at a con. Personally, I get along with just about anyone, but if you're shy, especially, you might feel intimidated making new friends, so try to hedge that bet a bit if you can.
Get a schedule and try to pick out the things you want to attend/see. If you are or aren't suiting, go to the fursuit parade. If you fursuit, join in, and if you are just watching, it's the best way to see most of the suits at the con.
If your con has a con suite, use it to get a snack to give an energy boost.
Ask permission before snapping pictures. It's just plain polite. Don't go into the headless lounge unless you are a suiter or handler. ESPECIALLY don't take pics of people with their heads/costumes partially removed as that is considered a faux pas.
Ask or give a gesture for permission to hug. Don't "glomp" hug (double secret attack hug from a blind side). The other person might not be ready for it, especially a fursuiter.
Don't "camp out" at anyone's table during the con. The dealers and artist alley folks are trying to make a little money, and unless you are buying something, don't block the traffic flow. It's OK to give a polite "I love your work" or some other suitable compliment, but move on unless the artist/vendor engages you in conversation. Also, if you are commissioning art, try to get your order in early so that you have a better chance to get your art finished during the con. Don't harass any artist you commission. They want just as badly to get your art finished as you want to get it. Showing up every fifteen minutes doesn't help.
Don't be rude to others. I'm sorry I have to even bring this up, but there are some pretty rude folks out there. Don't be one of them. The con is a place for fun and to relax as being a Furry. Don't impose yourself on others or make rude comments about their appearance or behavior. Mind your language and dress appropriately, especially around minors.
Speaking of minors, all cons maintain a strict policy concerning minors. No drinking/drugs, sexual contact (including language), or showing of sexually oriented art or products. If you are under age, it is not cool to try to cop some alcohol. You endanger the con and risk getting whoever gives you alcohol/drugs in major trouble, as in jail time. Also, if you get caught with drugs, no matter how old you are, you will be kicked out of the con, and probably be visited by the police.
Don't drink to the point that you become a liability. If you are going to get blitzed, have friends to take care of you. If you become too much of a problem, the con has the option of kicking you out.
Dress appropriately. This varies from con to con. Usually a collar is all right, but BDSM stuff normally isn't. If you want to wear questionable attire, ask the staff first if you plan on wearing it in public. Whatever you do in your room is up to you.
Don't trash your room or the hotel. The con is responsible, and if THEY get charged, you can be sure YOU will get charged and probably get banned, too.
I always try to remember to TIP my room cleaners. A buck or three on the pillow for them makes sure you get good service and it makes the staff feel loved.
Don't argue with hotel or convention staff. Use discretion and courtesy when dealing with the hotel staff. If Con staff tells you something you don't like, don't argue with them. If you feel you aren't being treated fairly, politely ask for someone higher up. It depends on what the problem is and where the problem occurs. Try to keep your request within the area where the problem occurs, like the dealer den, registration, etcetera. Don't ask for the con chair for every little problem. They have enough to handle.
If you lose something, ask a con staff member where the lost and found is. Stuff normally gets turned in. Even money, sometimes.
Stay hydrated. Especially if you are fursuiting, and double especially if you are fursuiting and at one of the dances.
Support the con's charity. Even if it is only a dollar. It's for the animals.
I'm tired and I am sure I didn't cover everything, but it is what is is. If you have a specific question, just note me on me FA page - Keefur.