Well, since you're just starting out, I would suggest NOT spending 500 dollars on something that you're beginning with. It's best to work your way up, espically if you just stop playing one day.
Nylon strings are more often used to play "classical" music, or classic rock, things like that. If you're looking for more of a "modern" sound like acoustic songs you may hear on the radio, then you'll want to get steel strings.
To tell if a guitar is good or not? You'd have to know and understand all of the parts on it. Neck, keys, pickups, hollow body, ect. Once you get into it though, you'll quickly pick up. Other then that, you want one that sounds good, yes.
If you change your mind and plan on getting some kind of electric guitar, check out the beginner package deals. That way, incase you decide you don't want to do it anymore (believe me it gets frustrating and many people quit) you didn't waste hundreds of dollars on something that will sit in your corner for the rest of it's life. Try a Fender Squire package, you get an amp, guitar, gig bag, picks and even a tuner I do believe; all for around a $150-200 (American coin).
As for a few good acoustic brands. If you're looking to spend top dollar, Ovation. Fender's are nice, but my god. An Ovation is an item straight from god's hands. Quality, and it sounds amazing.
Hope I helped somewhat. =)