We suggest coming in and looking at a few. We've got great bike models for every type of cycling and we can explain the differences and even arrange a test ride if you want, so you can feel the different rides.
In making a selection, it helps if you can tell us how you'll use the bike, where you'd like to ride and approximately how much you want to spend. If you're not sure, consider where you live and what the roads, paths and trails are like. Talk to friends who bike to find out what types they prefer, where they ride and what they recommend. (If you plan to ride with these friends, you'll probably want to get the same type of bike that they ride.)
Also, think about other purchases you make: are you a get-the-best, cost-is-no-object shopper or do you think of yourself as frugal? Do you like the latest high-tech gadgets or prefer simpler, more traditional designs. If you can answer some of these questions, it makes choosing a bicycle easier.
Keep in mind that cycling is a sport that grows on you. Many riders start with one bike and end up with a bunch, each ideal for its intended purpose. For example, an enthusiast will have an off-road bike for hitting dirt trails and a road bike for cruising on blacktop. If she's married, she might also have a tandem so her husband can join the fun. Or perhaps a city bike equipped with a basket for running errands around town.
Obviously, we're not suggesting that you start off by purchasing a garage-load of two wheelers. But, it takes some pressure off the decision process when you realize that no one bike is going to do it all. It's best to start with the bike type that seems best for how you'll ride now. And then, as your riding interests expand, there are plenty of other models you can consider.