redirecting to login site

Question

Hi,

i have a wireless access point, its open for everyone.
but, when they get connected to my network using their wireless devices, and start to browse (ie. www.microsoft.com), they won't get to microsoft website, but they'll redirected to my login site.

on this page, a user must fill in their username and password. after logging in, he/ she would be able to browse the internet.

any ideas how to do this?
what kind of approach should i use?

thanks!

---

AddinGanteng