Organic vegetables need 6-8 hours of sunlight each day. Choose a sunny spot. The land should be fairly flat, unless you plan on using all raised beds in a terrace fashion. If your location is on a hill, I recommend that your raised beds be enclosed in frames. Otherwise, a hard rain when the garden is not full of roots, could cause your precious soil to wash down your hill. Gardening on a hill can also be more work when hauling soil amendments such as compost or manure, as well as the harvest. It certainly can be done and in many parts of the world, alot of food is produced on hillsides. Just learn to work with it if your garden must be on a hill.
Do not plant close to a tree. Tree roots reach out at least as far as the branches and will compete with your heirloom vegetables for food and water.
Vegetables need water during dry times, so be sure you can reach your garden with irrigation. It is also convenient if the garden is not too far from the kitchen. I am much more prone to run out for a handful of basil, or beans, or tomatoes for slicing if the garden is near the kitchen. I have even seen people grow greens, and a few herbs in a flower bed near the door. While our major garden is 100- 200 feet from the house, I often plant a pepper and a few herbs in the flower bed just outside the door.
Our next post will be on whether to plant heirloom vegetables in a flat garden or raised beds. As for myself, we have some of each.