Welcome to the forum! So happy that cycling has struck a chord with you. It will soon (if it hasn't already) become quite an addiction.
Riding on the roads is intimidator but it really depends where you live. Some cities have a very bike-friendly infrastructure. Cars are used to having to share the road with cyclists and pedestrians and as such, it's a lot easier to get around by bike. Unfortunately other places (such as the suburb where I live) are not that conducive to commuting by bike and thus makes for a more difficult (and dangerous) commute.
As fas a commuting to work, everyone is different. I would say 12-20 miles is decent but again, it depends on they type of road you will be commuting on and how bike-friendly it is.
As far as new bikes, $500 can get you a commuter/hybrid bike for commuting. You should go to several different bike shops and find a reputable LBS and then decide on a bike At that price range, I would look at a Specialized Vita.
As far as locks, I work in NYC so a lock is a must if I were to commute to work by bike (which I don't). I've hears people say good things about NY Fahgettabout locks.
Anyway, welcome to the forum and Happy Cycling!