This is taken from the League of American Bicyclists, but it's a good start --
Answering these few questions will help you discover what type of bike is best for you:
•Why am I buying a bike?
•How fit am I?
•Where will I ride?
•What kind of terrain will I be riding?
•Will I be carrying anything?
•How much do I want to spend?
Types of bikes:
Road: Dropped handlebars and skinny tires, for racing or touring
Mountain: Flat handlebars with a wide range of gears, large tires and suspension for unpaved terrain
Hybrid/Comfort: Provides an upright position for a more relaxed ride
Recumbent: Have a reclined position and come in a wide variety of styles
If you answer those questions, you can have a better idea of what you should be looking at. Hybrid bikes are going to be the most all-around day-to-day bikes, and the most affordable, and sounds to me like would be best for your situation. Fitness/Adventure bikes are good for this as well, and will generally be better quality and more expensive.
If you're planning on riding mostly on the road, look at a road bike.
If you're mostly going to be on trails, look at a Mountain Bike.
In general, it's about comfort. If you'll be riding for long periods of time and mostly on the roads, a touring bike is a good idea. If you're someone who would prefer to stay as upright as possible, a cruiser is your best bet.
Good luck!