Nikon D90 Manuals page 23

xxii Quick Start
Press the button, then use the multi selector to move through the menus. Press
the multi selector left to make sure you're in the main menu categories. Then press down to go
to the Setup Menu. Press right to move over to the Format Memory Card option. Press the
button to choose it. A warning message will appear to make sure you really want to delete all
your photos.
At this point you don't have any photos on the card so you don't have to worry about deleting
anything. Select Yes and press the button.