In terms of a new camera, I would agree with what others have suggested, the D3100 is the best starter DSLR out there.
However, if your budget only allows a used one, then a D60 or D70 would be the next best option. A D90 will be a bit more advanced and the price of a used D90 might be close to a new D3100, so might not be suitable for your budget IMO.