Dynamic Dependent Drop-down selection menus

Hello,

Do you have a tutorial or video that would assist me in attaining this objective?

I know neither Ajax nor Javascript, and so am having a problem.

I have two <Select><Option>.....</Option></Select> menus on a page. The former is for ClassType, the latter for ClassLength.
But unfortunately, the available ClassLengths depend on the ClassTypes.

Therefore, I need the options in the second menu (ClassLength) to change dynamically according as to what is selected in the first (ClassType); and preferably before the Submit button is pushed.

I have tried Googling, and have seen examples where this is done by incorporating both JavaScript and Ajax, but have not spotted anything that is straightforward for me to implement.

Can your tools do this? And if so do you again have either a tutorial or a video? Or even an example I can try to adapt?

I will understand if you do not.

Thank you,
KB