Well, you kinda answer your own question... just break it down, and it will come:
You: "How do I have to build the query, so that it just just shows contact-ads..."
SQL: "select * from contact-ads";
You: "...people how have a special zip-code in their profile..."
SQL: "select contact-ads.,users. from contact-ads,users where users.zip-code = '$zip-code'";
You: "...The second table just has the nickname of the person..."
SQL: "select contact-ads.,users. from contact-ads,users where users.zip-code = '$zip-code' and contact-ads.nickname = users.nickname";
There ya go, that should at least get you where you need to go...