What SLR lense do you have ? My suggestions for Canon DSLR who shoot pictures seriously are as following:

10-22 mm EFS if you wanna go for wide angle if not so 17-40 F4L is another alternative

24-70 EF F2.8L for the middle focal range ideal for portrait

70-200 EF F2.8L for the telephoto range.

100mm F2.8 macro for the macro-shooting ( close up ) type.

These are the must-have lenses for Canon DSLR ( mostly ideal for the Rebel XT and 20D )