search form at admin/content I try to administrate a lot of content in Drupal 7.

How may I add a search form at Content administration page (admin/content)?
I need it so that I can easily find a node when there are hundreds of nodes.
You can use Views Bulk Operations, which allows you to replace that administrative page with a view. As you are using a view, you can add an exposed filter to filter the list of nodes by the title, or any other parameters you choose; the screenshot shows a view that can filter the node list by the author or the state of the nodes.

As far as I recall, VBO comes with an example view that can replace the administration page for the content; you just need to change the URL used for the view, and customize the view itself.