drop down boxes, etc.

i am creating a document that needs me to use drop down boxes. I know how to create a drop down box, but I need to also create a second drop down box which gives me choices based on the choice I made in the first drop down box. For Example, Drop Down box #1 choices are A, B, C, or D. If I pick A, the Drop down box under it would have another set of choices corresponding to A only. Same with B. If i choose B, a drop down box under the first gives me a set of choices relating only to B. The problem is, I don't know how to do it. Can someone please help me, thanks!