Are the best bits of Skyline still shut for logging? I'd avoid it if it is (although you can clamber over the blockages…).
I take a full Camelbak of crap for long away rides (stuff that emac mentions) + first aid kit, chain tool, tyre patches, mobile. Also better to have too much water and get the extra training benefit from the weight rather than have too little :p
Take it easy at the start to, Whites & Skyline share the same first climb, it's not too bad but can take a lot out of you especially on cold legs. Technically the riding is pretty easy (unless you go silly fast) but there are quite a few small drop-offs so ride some in advance for practice if you're not too confident on them.
Edit: For food I'd just take a banana, an energy bar or two and a gel or two for a bit of variety – you prob won't need to eat it all though (especially if you stop at the cafe between trails 😉 ).
Oh also make sure you check the trail map and take a note of the post numbers for the emergency exits and the ranger's phone number.