Entry is free and you're allowed to take your own booze. They've even laid on toilets, which is probably sensible, as the areas are open at around 8pm, and you are advised to be at your chosen vantage point by 10pm.
The Greater London Authority has brought in external excitement generators Jack Morton Worldwide to organise it all and, as if it was some sort of draw, BBC Radio 1 DJ Nihal will also be playing some pop records.
For more information, see http://www.london.gov.uk/get-involved/festivals/newyearseve, and if you're in the area it might also be a good idea to print out an entry letter for Gordon's Wine Bar, in order to ensure that you can get in.