> How to build a router on Linux box?

This is a bit like asking 'How do I build a nuclear reactor, I need to
have it done by monday' :)

You will need to sit down and do quite a bit of reading.
There are howto documents, there are books on the subject,
there are mailing lists like this. You'll need to spend time with
all three types of information source.

The book "Linux Firewalls", despite the title, is a good
source of informaiton on routing with Linux.

The book "Linux Routing" is also good.