Actually then if this is a filter for a search, you should look into actually making / using Kentico's Filters, you can define a filter for an existing repeater (the product listing) and that way when they select, it applies a "where" condition automatically.
Otherwise, if this is not saving information, then i wouldn't do a form. If you were, then you would want to.
if using a Smart Search:
if applying to a repeater or data source: