Hi Tamra,

Welcome to Nikonians!
The SB-24 supports Film TTL flash control and your D90 supports i-TTL flash control and they are not compatible with each other.
As stated above, you will be limited to Non-TTL Auto and Manual flash control.
For the most consistant results, I would recommend setting the D90 in Manual exposure mode and Manually setting the ISO.
In Non-TTL Auto flash control, set the Aperture based on the flash to subject distance using the distance scale in the SB-24's flash calculator display. As long as your subject is within the range on the scale, the images should be correctly exposed.
The same holds true for shooting in Manual flash control, though the flash to subject distance must match the distance indicated in the calculator display since the flash power is fixed.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty