Wow removing all the concrete must have been the hard part! Do you
have a plan for what you want to make out of the garden? Esp.
regarding soil it makes a difference what kind of plants you want to
grow...

Try and see if you can locate good source material, I just studied a
book that had something on the subject for 20 minutes last night so
that's where my knowledge on the subject comes from First you'd
probably want some kind of an analysis on the soil. You can ask a
professional or just take a good look at it and consult books/the
newsgroup. Eg. does it have lots of clay or maybe sand, makes a huge
difference. What kind of a place/where do you live, is it a former
river plateau/sea bed etc - so what kind of soil would be expected in
the area?

You can buy better soil or soil materials and put them on top, or mix
them with the top layer of your existing soil. The thickness of the
required patently good soil depends on the plants you grow. As you add
manure over the years the soil will improve, as more organic matter is
introduced (until you hit the ideal degree I suppose). At first you
could pick plants that don't have serious requirements.