seting up splash page like

I created a simple promo html page and would like to have it accessed first and then I will have a link on it to direct my users to the main site. But, I am not sure what/where to make the change on the site to do that..

For instance, if they type www.world.com   they will get directed to www.world.com/promo.html  and then I will have a link for them to go to the main page  www.world.com/index.php 

Thanks.