Simply putting it in a tangible form gives you copyright. Beyond that you can register the copyright formally for a small fee or put a simple notice on the page saying Copyright 2008 Name of Organization.
This is a link to the information and forms you need to register your software at the US Copyright office: http://www.copyright.gov/help/faq/faq-forms.html#software If you aren't in the US, you should check the site of your country's copyright office for the additional information. While registration of copyright is not necessary, it does allow you to bring a lawsuit against anyone who copies your software without permission. More information: http://www.copyright.gov/circs/circ1.html#cr