I have a Joomla site but I wish to move it to Drupal-6 site. How must i start altering existing url's? Must I add the redirection inshtacces or use any redirect modules?

One primary reason of moving may be the forum module in Drupal which Let me use.
I'd be utilising nodealias &lifier global redirect modules. During my site I am using SH404SEF to create web addresses(with .html suffix).

You are able to absolutely place a redirect within the .htaccess file and reference the pathauto produced path.

http://drupal.org/project/pathauto creates URL aliases according to title etc. after which put something similar to:

RewriteRule ^misc/about.html http://www.example.com/aboutus/ [R=301,L]

for every URL. (This really is best inside your apache httpd.conf apply for performance reasons.)

Or there's a module:

http://drupal.org/project/path_redirect

Which appears like it will just what you would like but I have not attempted it.