What time you want to get there, is really dependant on how close to the front you want to get (presuming you have a standing ticket). If you don't mind being a couple of rows back, just queue an hour or two before doors open, and head immediately for the side once you get in. If you want to get onto the barrier to get a handshake, unfortunately you are going to have to queue from crazy o'clock in the morning - people normally start around 3am. I wouldn't bother for your first show though! It's also a big venue, so you might be lucky if you aim for one of the sides.
Only other tips would be to avoid being right in the centre, as that's usually where the biggest crush and the most amount of assholes are (most people are decent, but there are always a few drunken louts at any show). And watch your head for crowd-surfers coming forward from the back to attempt stage invasions - this normally only happens on the last song or encore though - if you spot the bouncers moving towards you, duck and cover your head, as someone is probably about to land on you.
Lastly, a lot of people will be heading to this aftershow party following the gig - basically a club night with all or mostly Moz/Smiths tunes playing, so I'd recommend popping down if you can
http://www.wegottickets.com/event/423115
Good luck!