Talk about a baptism of fire - from no long haul ever to economy to Australia!
As my DH said after we flew separately to Sydney once and I went to meet him at the airport 'That was a long frickin' flight!', but he had the leg from LA to Sydney, which is hideous.
I don't do window seats on very long flights, because I get claustrophobic, I don't sleep and I like to walk up and down. If you suspect that that might be you too I'd advise to you get an aisle, because if the people between you and the aisle fall asleep for hours you'll be stuck. If you're like my DH though and can sleep anywhere, but all means take the window and slumber!
I buy a 1.5 litre bottle of water in the airport to take with me on board, or two of the largest small ones I can find. The crew will bring around water and orange juice quite regularly, but you need to drink more than they bring to stay hydrated IME. Drink more than you think you need to.
Take clean pants and a clean t-shirt in your carry-on, plus a full change of clothes in case you lose your luggage. It doesn't happen often, but it does happen. Also take anti-bac wipes for the tray table and armrests - they're always filthy and the airlines never clean them with antibac. Also take toothbrush and paste, facial wipes for freshening up and a small bottle of moisturiser for your face. Take reading material, an iPod or similar, and some noise-cancelling headphones if you have them. A good eye mask and neck pillow are a good idea too, as is a warm pashmina or something snuggly to put around your top half. The blankets on planes aren't washed after every use, so don't put them near your face.