Tickets for the Hyde Park event will go on sale on Friday 12 April 2013 at 9 am. Anyone who wants a ticket should be on the phone and ready before that time. If you put 'rolling stones hyde park 2013' in a search engine you will be shown a long list of ticket selling companies, but it's best to start from a recommended source.
You could try Telegraph Tickets, for example - they actually started selling some early, so it you click here you may be able to get some that way. I can't guarantee these are the cheapest possible, but it is an official service so any offers should be genuine.
Once the tickets are on sale they will sell out quickly (the Glastonbury tour sold out straght away) and after that your only chance is to get a resold one. Do be careful with any 'special offers' though - a lot of these aren't genuine. Good luck.