Well, the usual preparation for gardens includes tilling, adding new top soil and mulching. Depending on the type of soil you have locally and the kind of plant you want, you can further improve soil's PH value. Have you tested your soil yet? Sometimes a weedy garden doesn't mean the soil is bad. Most likely, you can much improve your garden by covering the very basics: drainage, top soil, and mulch, last but not least, water often in summer.

You have sand underneath of entire garden? Is it common in your neighbourhood? We use to have a lot rocks in the garden, what we did was simply plant around it. Few people in our development actually hired landscaper to redo the entire beds. It all depends on time efforts, cost, and your goal. You can try plant some shallow-rooted perennials.

Never tried makingcompost myself though.