dynamic query string

Hi list, I have my data distribuited across 3 tables, that I'm using left join to retrieve data from them. Now I need to make a search system in this tables and I like, if possible, that someone can send me tips in how is the best way to concatenate the query string in order to do the search. The users will have about 8 types of options to search, age (18 - 25, 25-30..), gender, nationality..... Thank's Rodrigo --