Think how you would want to be responded to if you were looking for a service. Be friendly, thank the person for contacting you, and let them know you'd be happy to see if you're a good fit for their needs. Obviously, you have to learn more about each other to determine whether that's the case, so offer to schedule a call to learn more about the project, or offer to discuss via email if they'd prefer.
It'd be smart, after you've learned more about the client, to ask how they found you. Maybe there's a revenue stream out there you're not considering.