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.
3) What do you have the time for? Some plants are very low maintenance like raspberry bushes or grapevines. Others are going to require a lot more attention. Decide how much time weekly you can put into gardening.
4) How long is your growing season? Tomatoes are a prime example of a plant you need to choose based on your growing season. Some varieties can be ready in under 50 days while others take over 3 months. Choose based on your growing season unless you plan on extending it with row covers.
5) How much sun does your garden get? Full sun, partial sun or shade? It's important to pick your plants based on the amount of sun the garden gets or they won't thrive.
6) What do you like? What do you buy regularly? You might have heard that one zucchini plant can produce 10 pounds of zucchini, but it won't do you any good to plant a whole row of them if you don't like zucchini very much.
7) What is local or cheap? If you can buy it locally grown and it's cheap you might want to pass on growing it if space is limited. Around here fresh corn is super cheap in summer. I can get it at the farmers market quite easily, plus I never have great luck with it. Same with strawberries. I use my garden space for things I can't get locally & cheap.
8) How much do you need? Do you want to can enough tomatoes for a winters worth of spaghetti sauce or just have enough for summer salads? Will you be making grape jelly or just snacking on grapes? This answer definitely impacts how many of each plant you'll need!
Bonus question) What makes you happy? I have a love affair with raw peas. It's quite bizarre, I know. Nobody else here likes peas and I really don't like preparing and cooking them. Nothing makes me happier though, then grabbing a few full pods from the vine and eating those little peas while I'm gardening. So I grow peas every year just so I can snack on them while I'm gardening. Also, growing up my grandfather taught me to garden in his city lot backyard where he grew green onions, cucumbers and tomatoes every year. I grow these every year because they remind me of him and that makes me happy. We also happen to really like those 3 and anything I have left over I give away to neighbors and friends....and that makes me happy too. I guess that's probably the number one reason I garden, because it makes me happy. Why do you garden?