Without really knowing you, I think you will want a hybrid, although a comfort bike probably will do. The comfort bike is likely to be somewhat heavier. Buy from a good bike store, where you will pay more than if you buy from a non-bike store but you will get a better bike (paying more, of course), advice on fitting (the most critical part of purchasing a bike!!!), and a bike put together by someone who know what they are doing. Plus, most shops throw in free adjustments to the cables and gear changers (called deraileurs), wheels, chain, etc.
I recommend that you check out Bicycles Inc. in Keller. I bought my current bike from Rick, the store manager. (I used to live in Southlake.)
Most of the bikes you see at decent bike stores are good bikes, and each manufacturer adds features to distinguish its products. Trek and Specialized are nice and give you excellent value for your money. Cannondale is known for its frames and you can get a headshock to soak up bumps, but you tend either to pay more or to get a bit lesser components. Other good bikes are Fuji, Lemond, and Bianchi.
For what you are looking at, the make of the bike is not that important - they all are pretty good. Just look for a good bike store and find a bike that you like.
I also recommend that you try out a road bike. If all of your riding will be on the road, the road bike will be much better once you get used to the position (there is a reason they all are so similar). If you will do mostly road with some unpaved roads or modest dirt trails, a cyclocross bike would be a good choice as it gives you the road bike position with strong enough components for modest off-roading.
If interested, you could check out
www.bikeforums.net and
www.cyclingforums.com.
Biking is a great lifetime sport, excellent cardio, so enjoy it.