Radha,
First, you need to add a Certificate plugin to Moodle. Use either Certificate or Simple Certificate.
A certificate must be inserted as an Activity in each course. The certificate will automatically insert the course title, student's name, date, etc. It is possible to upload custom images for the certificate border, watermark, signature and seal - that is done via the configuration page in the plugins menu in Site administration.
I often create highly customized certificates in which I add elements, change text, or modify the position of elements. I do that by creating a new folder in <moodle directory>/mod/certificate/type and adding my own customized certificate.php file in the folder. The new certificate type is then available in each course.
Floyd