New user His posts

How can i add to the new user welcome mail this user’s post’s (it would be only one, because the users on my site register and send there first post in the same form).

So the mail would look something like this:

Welcome new user!

username: name
password: 3234hi234uh4

This is your first post: http://www.blog.com/the_post_slag/

Any Ideas?

Thanks,
Ben