Sorry should have said, when you let it out, stay out with it, until it has done something (sorry for calling it 'it' but you don't say if its male or female). Lots of praise if it does do something outside. Try to ascertain what it has been used to doing, i.e. was newspaper put down for it, if so try doing this but restrict the area to a small place near the back door. Has it been used to a cage? They usually don't like to soil their own 'den' so maybe somewhere of their own to sleep might help.