Just take the S8 into town - a good stop to exit is Marienplatz, and there's tons to see within easy walking distance from there. Have sausages for lunch - I recommend Glokl am Dom for the Nurnburger sausages (when facing the Dom (cathedral) head down the right hand side and it will be on your right.) Or, if coming from MarienPlatz just head down the left-hand side of the Rathaus and take the first street to the left, the restaurant will be right in front of you.
If you like cars, the BMW museum is an easy jaunt up the U3 from MarienPlatz, and if you get the EU21.30 day pass all your travel will be covered with a single ticket. As a bonus you can watch people having their cars delivered and then driving them down what they call the "victory lap".
The Residenz museum is an easy walk from MarienPlatz and I recommend this highly. If shopping is on your list there are tons of stores in this entire area, and if you indeed are traveling between Thanksgiving and Christmas the Christmas Market is very nice indeed. If you get up to the Residenz, there is also a very friendly smaller market there where we bought most of our stuff.
Buying your ticket is very easy. When you leave the baggage/customs area follow the signs toward [S-bahn] and you'll see ticket machines on your right. There are English Language directions, and the only trick is to be sure you're buying the pass for that date, there are options to buy for future dates as well. You want the one for 2-5 people, and at least the last time I was there the machines did take US credit cards. If not, there is an ATM on your right just when you exit customs.
Once you have your ticket (you'll get one ticket for your entire party) you'll exit the terminal and then walk outside across a little plaza toward the station. In December there will also be a Christmas market there!
There are also ticket machines in the station if you miss the ones in the terminal, but the lines can be longer there.
You should be OK since you're traveling on a week day. When we were there on a weekend last fall there was trackwork on the S-bahn system and we needed to change trains at OstBahnhof, but that work seems to be limited to the weekends. I'm also not sure if the work is completed on the HauptBahnhof, but it was a complete mess last fall, but in any case if you exit at Marienplatz you'll miss that completely.