anything from a real mountain bike brand (i.e. not walmart trash) is going to be fine. just look for disc brakes (preferably hydraulic, but that's uncommon at your price point, though >>1520593
has them), and don't worry about how many gears it has. 21 vs 24 vs 27 is immaterial; they all have a bunch of overlap so effectively have a lot fewer gears. hell, my good bike is only an 11 speed. go to your local bike shops, try a few out, and buy from the combination of best bike and best staff.
you're also going to get retards telling you to get a used bike, but that's a terrible idea unless you can bring a friend who knows bikes to look at them for you to make sure they're not fucked.