Park at the O2. Go on their website and you can book it all on there (you can just turn up on the day and park I think, but obviously booking before guarantees you a space, and the queue to get in is also shorter/quicker). Costs around £20 but saves a lot of hassle.
Its pretty easy. A tip though, if you go up really early, you can park in the tube station / station car park for about £3 for a day, and it is easier to get away, but is usually full by mid afternoon for an evening performance