Driving into central London is total madness. (Traffic doesn't move any faster now than it did in the days of horse-drawn transport. Signposting within central London is meaningless unless you've got a map of London in your head. It's easy to stray into a bus lane - with a hefty fine - or get trapped in the wrong lane and end up miles from where you want to be). Leave your car at a station outside the charging zone and take the Tube. (Get a one day travelcard, for unlimited travel on tubes and buses; it will be much cheaper than paying for parking in central London).
If you insist upon entering the charging zone it will cost you £8 as long as you pay by midnight on the same day. If you pay on the following day it will cost you £10. If you leave it later than that it will cost you £60 if you pay the penalty charge within 14 days, or £120 if you leave it until between 15 and 28 days. Thereafter it's £180.
Full information (including methods of payment) here:
http://www.tfl.gov.uk...s/congestioncharging/
Chris