Rotating

I'm very new to modelling and still getting used to the tools, and I want to make a face at a perfect 90 degree angle. I'm wondering if there is a place where I could just type "90" for degree's, or do I have to rotate by hand?

Thanks

what program do you use. If gmax or 3dsmax select the object then click the rotate button then right click the rotate button and then on what ever axis you want type 90