The first thing you should do is start a dialog with the customer. Make sure you're on the same page with them. Agree on the plan, budgets, timelines, scope, priorities, etc.
Immediately after you are selected as a Copilot, you should receive an email from TopCoder with details about your next steps. Be sure to read them carefully and begin talking to your customer as soon as possible.
See details here for more info: http://apps.topcoder.com/wiki/display/tc/How+to+Compete+in+Copilot+Opportunities