I always give the same advice to first timers at Diablo:
1. For your first time, spend the money and rent a bike. They used to run the DH line of Iron Horses, but I think they moved over to Jamis. You want to enjoy your first downhill experience as much as possible and make it about challenging yourself, not make it a Felt demo day and miss out as a result of your bike not being up to the challenge.
2. I highly recommend the purchase of a full face helmet. By purchase, I mean do NOT rent a helmet from Diablo. For a modest price you can get a real nice 661, Rockgardn or Fox helmet (Bell's stuff is very uncomfortable IMO), and trust me when I tell you that Diablo's helmets REEK of the B.O. or riders past. It's almost nauseating.
3. Bring plenty of waters/gatorades and pack a light lunch (like a sandwich and chips). Since DH riding is not as much of a cardiovascular push as it is a workout on the leg, chest and bicep muscles you don't have much chance of getting cramps from lunch, and you will save a small fortune from buying food on the mountain.
4. Comfortable flat-soled shoes and nice riding gloves/Mechanix gloves are a big help, and you are better to wear comfortable fit shorts and a T-shirt instead of lycra. This is as much a functional mobility issue as it is a way to prevent heckling from the regulars.
5. There is NO NEED - I repeat, NO NEED, to have a Camelbak/Dakine bag with your water pouch, patch kit, tire levers, pump, granola, bad aids, picture of Grandma Babs etc. An average run is, with stops to get your bearings, maybe 10-15 minutes tops, so you can hold out on the juice box and animal crackers till you get to the bottom. If you get a flat, there are plenty of main ski trails that you can walk your bike to the bottom and fix it there. The backpack will inhibit your dexterity and be all over the place if you are hitting jumps and drops.
6. Never stop in the middle of a trail to analyze the situation. If you want to investigate a double black diamond trail or a feature, ditch your bike in the woods and walk along the side of the trail to get an idea of what you are in for, allowing incoming riders to have the right of way at all times. Parking in the middle of the trail will get you hurt or severely verbally abused, and it is straight up disrespectful.
7. Don't be afraid of black. For all first timers, I recommend an opening run of GREENHORN to DEVIANT to LOWER BREAKOUT to ALPINE. This gives a good, manageable introduction to smooth flow, technical rock gardens, tabletops and berms. Trails like DIRT MERCHANT, TRIPLE DROP, TEMPEST and VICTORY shouldn't prove too problematic at all, though generally some type of nasty name (ASYLUM, SLAYER, RIPPER, DECEIT) have some bark to go with that bite, and I might steer clear of them for a couple trips.
Apologies for the long winded response, but as a guy who started going up there on a Trek 820 with a rented helmet and now a season pass holder, I want everyone new to the scene to have some advice based on personal experience. You'll have a blast, I'm sure.
Unless you end up in the hospital.