Meta Data on individual pages

Hi,

I am currently developing a website and would like to add some meta data to individual pages ( meaning different pages have slightly different meta data). Although this sounds like quite a trivial task, at the moment I only know of one way to add the the meta tags, which is to change the header.php in the template. This then updates every page on the site so that the header is the same?

How to I have different meta tags for different pages?

Any help is much appreciated thanks

James