Create a a form for custom taxonomy terms

at the simplest level, you could put the options in a select list which would allow a user to ctrl + click to select multiple terms for your form submission. (For something fancier, use javascript/jquery/etc)

You can then write a return function that uses wp_query to filter on the terms (you didn’t specify if you were only looking for posts that have ALL of these terms associated with them, or at least one of them). See: http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters