Market a web-based app

I recently had a web-based app designed. It is ready to go! Seeking assistance with marketing it to two types of customers (Customer #1 = students/interns. Customer #2 = companies seeking interns). The success of the site depends on both customers joining simultaneously. Companies are required to pay to advertise their open positions and will not want to advertise without students. Students are able to sign up for free but will not want to do so if companies do not have positions posted. Will also need to discuss a plan (ie: focus on a small region and branch out later, method used to find both customers, pricing structure for paying customers - basic package, premium package etc). Please help.