Another thing to consider is how sure you are that you're committed to riding. If it's an experiment and you're not sure if it will stick, you might be better off spending less money to get a used bike or an entry-level new bike for maybe $300. If you decide you like it then in a year or two you can upgrade to something nicer and keep the old bike as a beater/emergency/loaner bike.

On the other hand, if you're really, really sure you're going to commit to this, it may make more sense to spend money to get a nicer bike up-front.

A "hybrid" is probably a good choice. A lot of companies are now making dedicated urban and commuter bikes that would be good to look at. 700x32 (wheel diameter and tire width, in millimeters) or 700x35 tires will probably be best, but if you start with fatter tires (700x37 or 700x40) it's easy enough to buy narrower tires and tubes later.