As a wine grower no single decision that you make will be more important and influential on wine quality that site selection. Many people already have their sites assigned to them, such as a family farm, but ideally the wine grower seeks a piece of land that is well suited to grow the type and style of wine that is desired. Sites are complex with many subtle features that will affect wine quality. The more study and effort you put into the finding the right site for your wine, the more likely you are to achieve the wine you want to make.
These resources offer excellent background information about the complex process of site selection. There is more in-depth material, such as Understanding Vineyard Soils by Robert White. Skilled consultants and extension educators can be very helpful. No amount of time or expense should be spared in the search for a suitable vineyard site.