(1) Enable Port forwarding of your router. Forward Port 80. This *might* be tricky or easy as hell, depends on configuration of router.
Do check out the make of your modem and the appropriate tutorial here: [*portforward.com/]
(2) Which OS do you use?
In Windows, I recommend XAMPP [*www.apachefriends.org/en/xampp.html], make sure you secure it properely.
In Linux, just install the Apache/MySQL/PHP packages. Just google LAMP + distro name, you'll find very easy to use tutorials. Similar with *BSD or Solaris.
In Mac OS X, it's a simple case of enabling Apache.
(3) You then need to copy the files, or install the CMS, and set them up. Look for their documentation.
Remember, Indian ISPs are largely unfit for hosting websites from home since of low upload speed. There's also a case of electricity bills and managing uptime.
If you want to really host a website, use professional hosting. Starts from Rs.250/year and worth for the uptime offered.
Hosting from home is good for developing web sites and personal file sharing.