For $500, you can probably find a 3-4 year old Cannondale (CAAD8), Trek 1.1-1.2, Specialized Allez, with Tiagra or 105 components that would be a good starter bike.
Peformance isn't horrible, but theyre kinda like Walmart for bikes. They sell mainly lower brand stuff (Fuji, GT, Diamondback), but they're not like shit you get at walmart/target/dicks. None of those brands are bad, especially to get started. Upgrade later if you like it.
Also, I'm a roadie, that's where my vote is. I've done MTB, and it's a blast, BUT, you always have to go somewhere to do it. Road bike I can walk out my front door and just go. How tall are you? I'll look around a bit tomorrow and see if any friends are selling anything end of season