For Example: You go to yahoo.com and you must enter your email address for a yahoo manditory newsletter before gaining access to the main page... It would be helpful if it was coded in HTML or a PHP script hosted on anther server.

It's possible to do but unwise. These people will just unsubscribe.

However, if you don't link to the rest of the site from your homepage then they will have to subscribe (which would then forward to the next page). This site uses Aweber autoresponder and has done something similar to what you're wanting to do (see below). Note: it does not force people to subscribe!

im not very clear about html but try making the default page ask for the user to input information, then at the end of the process, direct it to your main page