I don't get all fancy like a lot of bikers do, but I consider myself pretty good and like to get out at least three times a week. I usually never shuttle cause there just isn't much around here and I like the exercise and reward of pedaling to the top.
Anyway, I usually just wear basketball shorts and whatever tshirt I have on--sometimes it's too hot and I'll take the shirt off for the climb. I'm using some ratty old skate shoes at the moment--definitely have a pair of shoes just for biking cause they will get ruined. I really need to upgrade, though, and I definitely recommend getting some bike-specific shoes. I use platform pedals, but clipless is a possibility in the future, though I'd recommend platform to start off.
Essentials: spandex bike shorts! padded! Didn't have them for 2 years starting out and wish I'd had them the whole time.
Water of some sort, and lots of it! A camelbak is awesome, but I lost mine and the waterbottle's been just fine since, though I do miss the convenience.
Backpack! For both holding all your stuff (extra water, food, equipment, etc) and I really like it cause it adds a little protection for when you crash, which you will.
Bike tool+! Definitely always have a multitool--go to a bike shop and look around, maybe talk to the guys there to recommend one; it's always good to have more, but not always necessary, and know how to use it! The most important things to have are a spare tube/patch kit, tire priers, and a pump, or else you're stuck when you get a flat.
Helmet, definitely. Gloves, highly recommended. Gloves are good for the obvious reason of crashing and scraping your hands up real good, but also cause the oils on your hands break down your grips faster. I far prefer them, whereas some go without them, so it's something to consider. I rode without them for a while, but then really liked having them when I got them.
Sunglasses are very important--keep the sun out, but also dirt, water, sticks, leaves, bugs.
On the trail, always yield to hikers, horseback riders, and people riding up the trail if you're going down. Always tell people how many are coming down behind you if you're not alone. Try not to skid around corners all the time; hard to do, but it's nice to make an effort to make the trail last longer, especially if it's really wet or really dry. Try to keep track of other people you see on the trail, it can be really dangerous coming around a corner and some hiker's lollygaggin in the middle of the trail; I usually avoid trails that have lots of hiking traffic. Lots more, but all I can remember right now.
Technique: always look where you want to go. Constantly be updating your lines--pick one and stick to it whether climbing or going down; check out lines as you go up if you're coming down that same way. When going down, the faster you're going, the farther you want to look ahead, and never never be looking straight down at your front wheel. Your front brake is more important than your back: this helps with skidding out, which is bad for the trail and your control over the bike. Learn to get your butt way back out over your back tire and use 60-40 front to back brake, roughly. Brake before corners: braking in the turn lessens the tire's grip on the dirt. There was a really good thread on pinkbike that had tons of tips like these so maybe search for that one..
For maintenance, all I can say is clean your bike as much as you can (every ride ideally), and always keep everything all lubed/greased up. If it's not shifting well, get it adjusted sooner than later, as it will only get worse and your whole drivetrain will lose its longevity. Do a pre and post lookover of the bike to see that everything's in working order, again, the sooner you find problems, the less damage will be done.
That should be a good start; holla in a PM if you have any questions.
