Find someone who knows bikes to help you get a used bike. For $600 you could get something that was $1200 new, two years ago.

Don't worry about upgradeability. When you want to upgrade, get a whole new bike. It's about half as expensive to buy parts together on a bike than piecemeal.

As bbarend says, ride as many bikes as you can to get a feel for what you like. If you give some more details about what you like in a bike, you'll get some more suggestions of particular models to try.

Good luck!