Deciding what model of bicycle to purchase can be easy for experienced users but slightly difficult for newcomers. Basically you need to understand what you want to get out of your bike. Mountain bikes (MTBs) are good for what's in the name --mountains, climbing, going uphill--, because their gear ratios make pedaling easier. If you tend to stay urban, you may want to opt for a standard Mongoose, Huffy city bike, which are made for simple rides: good for speed but tend to put more pressure on your legs.
In Europe, bike shops are very common because the use of bikes in the country over vehicles; however, in the United States bike shops are around but not as commonly found. In America, while it is common to see children riding bikes, you will hardly find adults biking, because they are more prone to using motorized vehicles. Large retail stores, such as Wal-Mart, commonly have a selection of bikes, but they are often poorly engineered and therefore cheap. If you want a durable bike you would need to go to a bike shop and get a bike from a reputable company.