See the Express docs as well as the Node docs for https.createServer (which is what express recommends to use):
var privateKey = fs.readFileSync( 'privatekey.pem' );
var certificate = fs.readFileSync( 'certificate.pem' );
https.createServer({
key: privateKey,
cert: certificate
}, app).listen(port);
Other options for createServer are at: http://nodejs.org/api/tls.html#tls_tls_createserver_options_secureconnectionlistener