Option 1:
You can set one of your webapps as default. This can be done with the admin console:
Configuration --> sever-config --> virtual-server --> server
Then select your default web application from the drop-down menu. If your webserver port is 8080, you can start your application now by:
instead of http://hostname:8080/webapp
Option 2:
You could also deploy your default webapp to "/" instead of "/webapp" but I think the first option is more flexible.