crystal report by one click

hello friends,I want  application such that i have 2 combo box,one for displaying name of all databases of sql and one for displaying all tables in that database.and at event of selected index chaged i want to create crystal report and display on same form . I have taken 2 combo box on crystal report viewer but i dont know how to display data in crystal report viewer from combobox.Please help me,It is very urgent.