The soil under the concrete will all be compacted. So the first thing I would do is try to get that improved. Tilling is one option but you can also try using a broadfork. I would then add some compost if you have it and then a layer of wood chips or leaves on top of that. This will build soil life which will help keep the soil from being compacted again. I would then plant potatoes, carrots, and other root type crops that will help loosen up the soil. Nitrogen fixing vegetables like beans and peas would also be a great first group of plants to plant.