You can book tables at Oktoberfest by contacting the individual tents directly. You'll find the relevant phone numbers on the "Tents" page (http://www.munichnights.com/wiesn/wiesn_tents.htm).
If you do not have a reservation you can of course still buy beer. It just means that you will have to do a bit more work in order to find a seat. In every tent there are seats which are reserved from early in the day, seats reserved from about 3pm or so and seats which are free for all. You just need to find a free seat and you'll be sorted. There are no reservations for the outside seats so you just have to walk around and find a space.
In the next week or so I'll contact the various breweries and check if there's any availability left for reservations. Quite often many of the seats are booked up a year in advance.
