Validating fields

New to javascript and need a quick tip... Thanks for your patience...

1 - Field value:
I have a field setup as a drop down with 3 values (1, 2 and 3)... if the value selected by the user is set to 1, I would like a couple of fields in the same form to be automatically populated with a given value.

Any idea of the javascript code i need to create? Where do I need to put the code for this on the combo list field or on the field which value is automatically set? Do I put it in the calculate tab or validate tab?

2 - If the value selected in the combo list is 2, I would like a field F to be filled (mandatory)... What is the javascript code for this one?

Thanks for your help... This will help me get started and understand some basic concepts.