There are lots of tires on the market. Lots and lots! Every manufacturer has dozens of different designs, each available in a slew of different sizes and compounds. To get to the right one, you need to ask yourself some pertinent questions.
First up is size. Most obviously, your tires need to fit your rims - if you're using 29in wheels you've already got a slightly more limited choice, even more so if you're a 650b early adopter. But diameter is only part of the story. The width of tires is limited by how much clearance your frame gives you - most modern bikes will take a 2.35in without too much trouble, but 2.5s can present issues.
If you're XC racing then you'll be wanting light weight, which usually means narrow - look at 2.0-2.2in tires. For general riding we're fans of big tires, with 2.25-2.4 being about the mark - bigger than that is the preserve of DH and FR tires which are usually work well, due to the casing being forced into a funny shape.
Then there's rubber compound. This is a fairly straight trade-off between durability and low rolling resistance from a hard compound and grip from a soft one. If you're pushing the limits in corners you'll benefit from a sticky tread, if you're more conservative you might as well have a tire that lasts longer.
Like compound, your choice of tread depends entirely on your favored terrain and riding style. If you want to roll fast on hard terrain, a shallower, closely-packed design will work well. If you want to corner hard on loose or soft trails, look for tall well-supported side blocks.
Then there's behavior on the limit. Tires with square shoulders tend to lose grip quite suddenly, more rounded profiles give a bit of warning. Similarly, some tires - usually, but not always, those with open areas of tread between center and side blocks - prefer to be quickly cranked over in corners. Others - often with a continuous tread across the width - are happy with a more subtle approach. Be honest with yourself and decide which is for you.
Finally, if you want to run tubeless it's a good idea to choose tubeless ready tires. Full UST airtight casings tend to be heavy, but tubeless-ready beads on a normal casing that relies on latex sealant to hold air works out lighter than tubes as well as offering a suppler tire for better grip and of course puncture resistance.