There’s a couple factors you’ll have to judge as we don’t have enough information yet to answer your question. Since it used to be a garden you might get lucky and only need to add some nutrition to the plot.
You can send a soil sample to be tested, or you can buy a little $15 testing kit and get a rough estimate of current nutrition.
Check how water absorbs into a little pot of the soil, it might be too heavy in clay, or maybe too sandy.
As far as general ideas, I like to run any available woody material through a chipper and add that to the soil too. And definitely agree with @99PerCent on the compost