Oyster cards can be purchased from the ticket desk of any Underground station. (Although, as you'll probably see from the rest of this post, it's probably not worth bothering for just one day of travelling around). You pay a deposit of �3 and then charge it up with as much as you like for travelling around. If you'll be travelling (within the central London area) before 0930 on a weekday, you'd need �6.70 credit on your card. If you'll only be travelling outside the weekday morning peak period, you need �5.10 credit.
The alternative (which might well be better for a single day of travel) is to buy a One-Day Travelcard (for Zones 1 & 2). That would cost you �7.20 if your journeys included travel before 0930 on a weekday, or �5.60 for travel outside the weekday morning peak period.
One of the best ways to see the sights is to take a river bus. (I recommend the short journey between Tower Pier and Westminster Pier; you get better views of some attractions than you do from land). If you show a Travelcard you'll get one third off the fare. (You don't get the discount with an Oyster card).
Chris