When you first start out in photography, you will have two ways to earn money- basically two business models. The first is selling photos. The second is selling your service as a photographer.
If you are going to sell photos, this means that you will speculate on what might sell and go out and take pictures of it. Then you take those pictures to your market and try and sell them.
If you sell your service as a photographer, this means that you will be given an assignment to take specific photos of something (either personal such as a wedding, or commercial such as product shots). These photos will usually have limited value to the client who has hired you (who else would want a photo of Erin and John's wedding?)
Either way you go- the number one key to being successful in photography is to know about marketing, pricing, negotiations, and having good interpersonal relationship skills. These are the keys to any good business. Without these, you may be an awesome photographer, but if you don't understand business, you will probably never make it full time.