To add more basic info to DJ's
Yes, opening weekend is a madhouse. If you can be there the Monday after opening weekend, then getting a seat in a tent from just after Lunch to 5:00 PM should be no problem. Saturday and Sunday it is less likely...but you never know. This year you could have gotten a seat in several tents on Saturday as attendance was way down (security concerns I believe)
You are allowed to enter a tent unless it is filled to capacity and they shut off entry. You can wander around and look, but cannot get a beer unless you are seated at a table. A few tents do have stand-up areas and outdoor biergartens though. Tables are generally reserved, but as I mentioned, if they are not reserved during the time you are there, then you can sit, but not past the reservation start time. Waitstaff will serve you, Beer by the Mas (1 liter), wine is usually available, then some menu is offered. Pretzels are a cheap option, chicken, Ox, and pork shank the likely meat options. People who reserve a table basically agree to purchase 10 beers and 10 meals, so if you try the tact of begging a spot at an underfilled table, offering to take on some of the cost, or buying a few beers is a good tact.
As DJ mentioned, standing on the tables is Verbotten, but standing on the benches, loud singing, and toasting your table mates with clangs of the glasses is expected. Trying to steal glasses is strongly frowned upon and you will get caught.
As for other biergartens, they will be busy, have bands, and almost as much fun as a tent. Not the same experience, but still one you need to have.