Sign up for an Office 365 Developer Site

1. You may already have access to an Office 365 Developer Site. If not then click on Create Account

2. You’ll have to wait for your Developer Site to finish provisioning. After provisioning is complete, refresh the admin center page in your browser. Then, click the Build Apps link in the upper left corner of the page to open your Developer Site. 

3. Now click on 'Build an App' tile. On the right side you will be asked to add the Napa Office 365 Developement tools. Click AddIt as shown in the figure below.

4. That's it. Now after you have a development environment and an Office 365 Developer Site, you can start creating apps for Office and SharePoint. Have fun

Reference: http://msdn.microsoft.com/en-us/library/fp179924.aspx