first things first ive never been on a plane before ... is there anything i need to know?
Bl00dy h3ll, never been on a 'plane. You sure pick a long haul for your maiden voyage.
As Chris says, it rather depends on whether you are turning left or right after you board. First and Business classes have good seats-cum-beds these days and you might even get a good night's sleep. Cattle class? forget sleeping, make sure you have a good book and your iPad fully charged.
Either way. My advice - as someone who used to commute long-haul on a weekly basis - is dress comfortably, no shirt and tie, no suit, sitting in even the most comfortable seat for nine hours is a pain. Slip your shoes off before take-off as well. Don't wear anything white - you will be guaranteed to spill food on yourself hunched over the fold-out table.
I once took two junior colleagues to the States. One spent the entire flight opening and playing with every freebie supplied, trying every snack, drink, meal, magazine, movie, music. It was like having a boisterous four-year-old with me. The other opened his book while we were still parked at LHR and didn't take his nose out of it, refusing the horrible airline food, not moving except to turn his book's page, until landing.
Everyone is different
Enjoy.