1.how can i insert an image to my site and how can i add link on it as when i hover on it then i go to the destination of link.
2.how can i put some downloads on my site.
3.how can i change the header of the site.


1) There are tons way for inserting an image, you can include your image path in the html code
Only local images are allowed., or download image field, which allows you to upload image from your local machine.
2) I assume you are talking about modules, place your modules at sites/all/modules (create one if you don't see this folder)
3) Go to the admin panel and go down to theme, click configure for your current theme. You can change your header color and image there.

Most answers are in the handbook( http://drupal.org/handbooks ).

Good luck