Are you talking about a "nursery", where you want to start with seedlings and grow them yourself, or are you talking about a "garden center", where you want to bring already grown plants in and sell them?
If you are serious about starting a nursery you should go to the local ag. extension office. They will have lots of good information. You need to carefully examine soil and water. I have seen more than one LARGE nursery have to relocate because of high salt levels in their water. This can cost BIG money so check it first. Certain plants need certain types of soil and different plnts ship better with clay or sandy soil. Labour becomes important in nursery work so make sure you can manage people. Your local extension service can also help with a marketing plan. I have worked in the nursery trade but don't currently have a nursery because I can buy plants much cheaper than I can grow them. I have seen a number of nurseries go under so make sure you do your homework before you start.