how do I force someone to visit my website when they join my wireless network?

No, it's not done on a router.

A RADIUS server is used to do this.

What you will need is a proxy server, also you might use a AAA server for authentication. What will happen is that everytime someone tries to connect to your wireless or wired network, the router will route the signal to the proxy server and the proxy sever will push a web page asking to input a username and password, once this is verified the proxy server will let you get into the network, not the router. AAA server is used to add more security to the authentication and authorization.

If you run a DD-WRT enabled router, you may be interested in http://www.dd-wrt.com/wiki/index.php/Chillispot

You can use "Captive Portal" to redirect users when connecting using your network.

What it does is redirect the users when first connecting to the internet to a validation page.

But the captive portals are usually used to validate your network users, I'm not sure if captive portals can be used to redirect them to your home page first but you can give it a try.

There are many captive portals free or paid out there, use search engines for finding them.

only they can make a page their home page...as far as "forcing" them to visit your page...no way to really do it...you can ask them to do so, you can entice them with something to bring them to it...but unless they want to visit....they wont. if they are on your network, jsut ask them to visit, tell them what its about...make an offer if they visit on a certain day or something....