select data from a drop down box in a form

I like to select data out of a drop down box in my form field from a mysql database. While I do that I also like that it reloads the page and put the relevant products of that drop down box in another field. Ex: Category 1 Category 2 Category 3 are in the drop down box and when I choose Category 1 I like to display another field which displays me all the products which are in that categorie. I know it can be down just not to sure how?