One ticket equals one tube ride, which is great if you're going one place (and you know how to get there). Otherwise, buy a one or several day London tube passes or a credit card-sized Oyster card, which gives you a set amount of rides for at least a week, works on central London buses and segments of National Rail trains, and is hard to lose. My one-week Oyster pass was about 22 pounds in March 2006.
Small LED signs on station turnstiles are green if you can go through and red if you can't; tube workers stand patiently by and point that out, and they'll help if you can't get through.
When you do pass through the turnstiles, you'll first feed your ticket into a slot or pass your Oyster card across a pad; the ticket pops back out -- hang on to it because you'll need to perform the procedure in reverse to get out of the train station at your destination.
Please go on to the next page to learn about finding tube tunnel directions.