There are a number of threads on here about Oktoberfest from over the years. Definitely do a google search.
Where to stay: There are a number of places right along Schwanthalerstrasse that is a short walk to the Wiesn. I've stayed at the Courtyard by Marriott, and it's perfectly fine. At least as of a few years ago, it was actually a very good use of Marriott points (which is pretty rare for that shitty program).
Tents: There are more than a dozen tents. Each of the six Munich breweries has its own tent. And then there are other tents that aren't directly affiliated with a brewery, but still only serve one brewery's beer. There's also a wine tent.
Each tent has its own vibe. Hippodrome is the smallest and coolest. It's where you're most likely to see a celebrity. Hofbrau is heavily antipodean. Lowenbrau is heavily American. Augustiner tends to be kind of low-key.
Seating: You can only get a reservation if you have a party of six or more. So if it's just two of you, you just go in and grab a seat where available. It's all long picnic-style tables, so you'll be sitting with people. It's polite to ask the others already sitting at the table "sind diese Sitzen frei?"
You have to be seated to order; there's no bar you go up to. You'll see "Reserviert" on some tables with a time, which means that it is reserved after that time. Up to that time, you can sit there. If/when the reserving party shows up, you have to scram (though they'll often invite you to stay if there's room).
Honestly--the sitting with strangers from around the world and striking up increasingly drunken conversations is the most fun part of Oktoberfest.
Attire: You gotta wear lederhosen. Your lady should wear a dirndl. Everyone else will be wearing that. We were getting dressed and Mrs.LL said "I feel stupid wearing this." Of course, we started walking to the Weisn and rounded a corner and everyone was wearing lederhosen/dirndl, and she felt a lot better.