Oyster cards are a minimum of 16.30 GBP (that's $18.65 as I write in spring 2006) and you can recharge 16.30 GBP at a time. You'll get a deposit and any leftover cash back when you return the Osyter pass at tube stations.
You can buy paper bus tickets by the day or by the ride in tube stations. These are not a snap to find (another good reason to get an Oyster card). A one-day ticket is about $4. Londoners think bus tickets are spendy because the government wants you to use Oyster cards. Perhaps; Oyster cards *are* handy, though.
When you get on a double decker bus, swipe your Oyster card over a card reader near the driver (he sits on the right) or feed your ticket into the reader's maw (you'll get it back -- keep it; you can be fined for riding without a ticket). This is so easy, and London bus drivers so patient, that you'll figure it out the first time you board a double decker bus.
Go on to the next page to find the London bus routes and bus stops near London tourist attractions with London bus maps.