It's hard to say which type of tickets will be available to you because you've not stated where you're travelling from.
There are some types of ticket which are common to all rail operators and other which are only offered by one particular train company. Most train stations can sell you a 'walk-up' ticket which includes a London Travelcard on the same ticket but you might still find it cheaper to pre-book your rail tickets and buy your Travelcard separately. (To get the cheapest bargains, you usually have to book well in advance. However, some discount tickets are available up until 1800 on the day before travel, so it could still be worth checking).
Go to
http://www.nationalrail.co.uk/ and enter your journey details. Then select the option to show fares. Then select the further option to show single fares. (With advance purchase tickets it's often far cheaper to buy two singles than a return).
If you find some bargain fares, a Travelcard (for unlimited bus & tube travel in Zones 1 & 2) will cost you �4.90 per day at weekends. Other Travelcard prices are here:
http://www.tfl.gov.uk/tfl/fares-tickets/2006/o ne-day/travelcard-day.shtml
Chris