I would like to give special promotion for the first 100 posts in my blog? Can anyone tell me how to do that?

Here is what I would do:

Run an SQL query like so:

SELECT * FROM `wp_posts` WHERE `post_status` = 'publish' AND `post_type` = 'post' GROUP BY `post_author` ORDER BY `post_date` ASC  LIMIT 100

The above will give you the first 100 authors to have a post published. or:

SELECT * FROM `wp_posts` WHERE `post_status` = 'publish' AND `post_type` = 'post' ORDER BY `post_date` ASC  LIMIT 100

this will give you the first 100 published posts