I have many site collections, where I want to add a webpart page named "reporting.aspx" and insert a developped webpart (c#) in it, How can I create this feature and activate it automatically ? thank you.
The best way you can add page with your web part is using Module.
I am sharing some links here like SharePoint-deploy-webpart-page-using-module and Quick-tip-provisioning-web-parts-to-a-page which shows you how to Create page using module.
Add this module in one feature and activate it in all site collection where you want this page.