If you are travelling in by train, most networks will also sell you a Travelcard for the tube in advance too ..... and this will save you a little money. If travelling in by another method, do as already suggested a get your Travelcard at your first station .......... don't even think about buying single tickets as will cost you a fortune! The Transport for London (www.tfl.gov.uk) website will let you plan your journey with specific times if you need to - especially if for an O2 event so you know when your later trains back are. lso worth knowing is that Travelcards and sometimes also national rail tickets will get you discounts on various attractions etc in London - this is all on line too at
http://www.daysoutguide.co.uk/