Bills fan here..being your first NFL game, let me give you some quick pointers.
1) Okay to wear your Patriot gear, but if the Pats go up by 21, don't start talking trash in the stands or after the game..that goes for any fan in an away stadium.
2) Say hello and be pleasant to the folks around you in the lots and in the seats close to you from the get go...then you can start to talk a little trash as you establish you are a normal person who realizes this is a football game , and not some ascertion that you are somehow superior as a person cause the team you root for is so damn good.
3)If a drunk chucklehead comes up and starts talking trash..walk away..again happens in every stadium, not worth a fight( and yes this hapens to me in Foxboro as well..very rarely but it happens).
4) root like hell for your team, just don't talk chit on the Bills while doing that.
5) keeping warm..the old addage on layers is always spot on. The person who recommended NON-Cotton..perfect advice, especially if there is any chance of precipitation. Just remember,once you get cold, very hard to get warm again. If you have plenty of layers on, make sure you take some off as you walk from the tailgate to the staduim..if not you can begin to sweat, which is not a great thing when are going right back to being stationary for the next 3 hours.
6) Do not time your use of the facilities and concessions to end of quarter, half, after punts etc. You have to gamble soetimes on missing two plays or so during a drive, but trust me after a punt you will miss the whole next series unless you are first in line.Get two beers everytime..and make sure you get all the beer you want in the 3rd qtr.