It depends what you mean by very shallow soil. What is under that soil? Solid concrete that won't drain properly, or just sand stones and gravel etc.? If the last then Leggi has the best approach; use easy alpines or bulbs. If you know nothing about alpines just look in the garden centre. Anything you get a lot of for your money is an easy alpine in general. Sempervivum, Sedum, Erinus, Dianthus, Campanula, Armeria (Thrift) etc. Several small bulbs should be OK, such as Grape hyacinths, crocus, snowdrops, the smaller alliums (onions) eg. chives or ornamental speacies. Maybe liriope or some of the coloured leaf sedges. Just look for the smaller grass like plants in plant sales.