http://journeyplanner.tfl.gov.uk Transport for London's online journey planner - select cycling option and deselect the other transport modes.

http://www.tfl.gov.uk/roadusers/cycling/11598.aspx Transport for London site where you can order the excellent Local Cycling Guide maps (esp 10 for Tooting and 14 and 7 for central London)

http://www.lcc.org.uk/ -  the London Cycle Campaign website has much useful information. Check out details of your local borough branch too.
http://www.wandsworthcyclists.org.uk/ -  Wandsworth's branch of LCC.
http://www.mertoncyclists.talktalk.net/ -  Merton's branch of LCC

http://forum.ctc.org.uk/ -  the forum of the CTC (Cyclists' Touring Club), has a mass of useful information and a good place to ask questions (though search to see if the topic has been covered before!)
http://www.sustrans.org.uk/ the leading sustainable transport charity

CYCLE MAINTENANCE
 http://bicycletutor.com/ is a good site for free online videos of cycle maintenance (a wide range of tutorial videos can also be found on Youtube)
  http://www.parktool.com/ bicycle tool manufacturers, have a comprehensive, free online manual
London Cyclist, the magazine of the London Cycle Campaign, has details of maintenance classes across London.

INSURANCE
The London Cycle Campaign and Cyclists Touring Club ( http://www.ctc.org.uk/ ) include public liability insurance in their membership.

Both organisations can suggest insurers against theft, or it may be covered or be able to be added to your household insurance