Setting up .htaccess

Hi

i’m redeveloping a site for a client.

Their current site is written in .asp. I’ve rebuilt in WordPress as they need a CMS.

We’re repointing the DNS to our hosting environment.

My question is – I need to set up an .htaccess file (I think this is the best way) to redirect all the old URLs to their new equivalent. Some of the URL structures have changed so it’s not just a case of changing .asp > .php. Soo… where should I upload the .htaccess file – does this need to go in the root directory OR does it need to go in my wp-theme folder?

Thanks