maheswaran,
Google does offer a site search option which has up to 5000 web pages for $100 a year.
This would be the easiest and considering your time to develop included it may be the most cost effective solution.
There are other solutions as well - I am unsure of how effective or customizable they are, you can find a list here.
However, if you are planning on doing it yourself, you will need to do the following:
Design a "spider" which will search though your site for text, and index the text (the easy way is to copy everything between the <title> and </title> as well as <body> and </body> tags).
You will also need to read up on Full Text Search to query the database.
I hope this helps a little... I would recommend using one of the existing products for ease of use.
