Add a field to Your users table, for example [acc_active] and make it default to false.
Then in the mail body:
<a href="http://someserver/activate.php?account_id=xxx">Activate Your account</a>
In the activate.php get the account_id from the QueryString and reset the [acc_active] field to true for the user corresponding to account_id.
Then redirect the user somewhere else, or just display a message saying that the account has been activated.