Build Apps for Office and SharePoint 2013 on Office 365 preview
https://msdn.microsoft.com/en-US/office/apps
Sign up to get your own Office 365 developer site domain. This will create Exchange, Lync, and SharePoint on Office 365. From the Office 365 Preview admin center, you can view all the services created under your Office365 domain.
Once all Office 365 services have been created successfully, you can click on Admins > SharePoint to launch the SharePoint Administration Center to view all SharePoint services.
From SharePoint Administration Center, you can configure all the related SharePoint services such as InfoPath, BCS, term stores, search, etc. You can also view all your site collections.
Click on a site collection url to lauch the site collection. Install and deploy "Napa" office365 developer tools as an app on your Office 365 developer site to start developing SharePoint apps.
Launch Napa as an app from your developer site, (This requires IE 9), you can click Add a new project or edit an existing project you have developed. You can then update javascripts, html, client side code, then click "Run project' icon at the bottom left on the page to package and deploy the app to your site. Click "Lauch App" to run it.