The quickest way is to cycle, which takes around 25 minutes, quite a simple route going up all the main roads (i know a few have cycle lanes).
The next quickest route is to get the train to Blackfriars, which takes around 40 mins.. it takes about 15 mins (included) to walk from Blackfriars to the strand. I'm not sure how much trains cost.
The next one is bus, which takes aroun 45-65 minutes depending on traffic. You can get the 68 and 176. The bus is £1 each way with Oyster.
You can't really get the tube as Denmark Hill has no tube stop. It will cost more if you get the tube from Blackfriars to Temple, so it's not worth it unless you are in a rush!
An Oyster is worth getting anyway, even if you don't think you will use it much.