Schwinn Mountain Bike

What you should be looking for to start out is a good bike shop. They can quickly narrow down what you can get for your budget and ability.
Things to look for include proper fit, a front suspension fork, maybe a rear suspension post, and a good brand name. The better known brands include Trek, Gary Fisher, Giant, Specialized, Fuji, and Canondale. These brands have been around awhile and with good reason. They have good warranties, and are sold through bike shops. A bike shop will save you money long term. The service alone will make up the difference in cost between a department store and a bike shop.
You are going to love this sport! But, remember there is personal risk involved. I highly recommend a helmet. I’ve wrecked a pair, broken a collar bone, gotten some scratches, but never a concussion. That’s a really big deal. A helmet is the cheapest insurance policy you can get.
A good pair of bike shorts, a pair of padded bike gloves, a good pair of bike shoes, and at least 1 water bottle with cage would be a good place to start. If you start getting rough, you can get body armor.
Most of all, find a group you like riding with and start riding with them. They will make you a better rider. Almost without fault, bicyclists are some of the most generous people with their time, advice, and knowledge that you will ever meet.
Hope you have fun!

Without a budget I couldn’t say what would be a good bike for you. If you XC riding as you indicated best value would be a hard tail for around $500 to start. This will insure you get a bike that can handle the riggers of XC riding without falling apart.
As for gear: Padded shorts, gloves, sport glasses (not dark so you can is in the woods) Helmet of course. Once you are ready clipless pedals and shoes. If you ride in cool or cold weather, windbreakers, warm gloves, head band, tights or riding pants, shoe covers etc.

You need to get a bike that is the correct size for you and there are so many different levels of mountain bike and prices that it would be had to suggest any one model. The best thing to do is go to the local bike shop and see what they have. You do not want to buy a bike from the department store. Take a look at the Diamondback, Trek, Cannondale, Giant, Specialized, Fuji, Raleigh, and a few others.

Let’s see. A bike, Helmet, good padded shorts, shoes and clip-less pedals( the new Time pedals are sick), and some glasses. A place to ride would be nice. Peace