whatever land you can take.

some cities have programs that allocate park or unused land for community gardens. some even give you a small budget to build infrastructure like beds or buying dirt.

grow staple foods that have long storage life: squash, pumpkins, carrots, rutabaga, potatoes. these can stay on your shelf for 3-8 months if stored properly. personally I have about 12 (3-5lbs each) spaghetti squash sitting since harvest in November that will be fine until about August.

secondary are things you can freeze or dry: squash, peppers, peas, green beans, Lima beans, kidney beans, cabbage, beets. I dry most of these and toss them into soups and ramen.

tertiary are foods you can process and preserved through canning, drying, or freezing: tomatoes (sauce or breaded), okra (breaded), etc..

your diet will change, but you'll feel good about what you're cooking because you grew it.

good luck!