With a 500 dollar budget I would highly recommend going craigslist and buying a used bike. If you don’t know anything about bikes or mountain bikes, give someone who does a beer and take him/her along with you.
My first bike was a 2009 Gary Fisher Piranha, which I purchased for 250 dollars. It was a 2009, and had 3×10 sram gearing hydraulic disk brakes, and an air fork with a lockout. Way better than any new 500 dollar bike. I then sold it for 350 and bought a 2013 Trek Superfly for 650 dollars.
So, I spent 550 dollars total and now have a 25 pound xc hardtail 29er that retailed for two grand. It’s basically brand new. If you wait long enough you’ll find a deal, even if you’re looking for a small or an xl frame size. I was also willing to travel a couple of hours to get the Superfly, but I couldn’t be happier.