1) What grows in your gardening zone? No matter how hard I try I will never get avocado trees to grow in my gardening zone and fighting it is pretty much useless. Save yourself the disappointment by finding out your gardening zone and plant accordingly.
2) What do you have the space for? Unless your garden is really huge, you'll have to consider space when deciding what to plant. Some plants take up much more space then others and overcrowded plants rarely produce well. You can find spacing requirements in seed catalogs and plan accordingly.