Is there a place at the airport I can buy a ticket to Garmisch, or do I need to go into Munich to buy it.If there is , what sign would I look for?
There is a DB ticket counter in the central area between the two terminals, over the S-Bahn station. It's at the Terminal 1 end of the area, near the meeting point. Look for the red "DB" symbol.
However, if you are leaving the airport after 9 AM (anytime on a weekend), you can buy a Bayern-Ticket (€19 for one person, €27 for 2 to 5 people) at one of the ticket automats trackside in the S-Bahn station. This ticket will cover you for the S-Bahn into Munich as well as the regional train to GaP.
If you will leave MUC before 9 AM but take a train to GaP that leaves after 9, the least expensive way is to buy MVV tickets from the airport into town, and a Bayern-Ticket to use after that. Or, if you want to be on the regional train that leaves the Hbf at 8:32, buy at the airport an MVV Gesamtnetz (entire network) Tageskarte (day ticket) for €10 Single, €18 2-5, and P-P ticket(s) for Tutzing to GaP (€10,90 pP). The MVV Tageskarte will cover your travel all the way to Tutzing.
I know this is all very complicated. If you tell me how many people in you party and when (day-of-the-week and time) your flight arrives, I can better advise you.