Reservations to the Oktoberfest are not necessary and entry to the grounds is free, however they can make the experience a little more relaxing, especially if coming on one of the weekends.
Reservations during busy times (which is really the only time you would need them) are however not easy to come by.
Generally there are two different reservation blocks per day. The first one normally starts at around 11am and the second at around 5pm, but this will vary depending on the tent.
Morning sessions are much easier to come by but the party at night is normally better, not to mention the fact that when the evening reservation shows up you have to leave your seats and find new ones in the general admission section, which is no easy task for a whole table!
When you make a reservation, you will generally be forced to buy vouchers for two "Maß" (1 litre of beer) and one half chicken. Most people will drink at least two Maß, and eat at least half a chicken, so the reservation ends up basically being free.
It is not possible to reserve for just one or two people. Normally have to get together at least a full table (10 people) to make a reservation, although the number is sometimes less for the smaller tents.
Beware of companies trying to sell reservations on eBay or other such sites for hugely inflated prices. Some agents are however reputable and will be able to use their connections to get you reservations at busy times for a reasonable fee.
To make a reservation you can contact the tents directly or use an agent to organize your tickets.
As a tourist, you already have a head start on most locals who have to work during the week, so unless your time is really tight, the best idea is often just to get to the tents early and take a seat in the unreserved section.