The question you ask has been tackled by entire books and entire websites :)
There are many plants (including citrus!) that can grow in potted containers year-round, even in northern climates.
There are many plants (check out
www.parkseed.com) that can grow in pots, hanging baskets, etc. and are suitable for even city and apartment cultivation!
My backyard is measured in square feet, NOT acres, as well -- but I'm not about to give up :) I figure a little food is better than NO food. Besides, I'm learning how to grow food, which will make me more useful later on (I want to have skills BESIDES 'computer programmer' -- which might be much less useful in 2, 5 or 10 years)
Being "more useful" might help me & my family to survive, but it will also help me to help others, which is even more important.
I have dug up about 650 sq. feet of my backyard, which even more to come -- I won't have much grass in my backyard for long. I have a front yard, and I can always mow the grass alley behind my house, if I miss mowing the lawn :)
Actually, I already mow the alley all the time -- I have a bagging mower, and I harvest the grass for my (huge) compost pile. It's free organic matter that no one wants, that I can turn into my own potatoes and tomatoes! What a deal! :)
I pretty much don't throw away anything organic. Sticks, leaves, etc. go into either the compost pile, or one of 4 garbage cans that serve as compost bins.
Matthew