You have to BUY it. If you are a student, you may be able to get a discount.http://usa.autodesk.com/autocad/
We do not support illegal software here at Tom's
Also if you just want to play with it, maybe one of the FREE autocad type programs would be helpful to you.
Google link for "Autocad alternatives".http://blog.cometdocs.com/10-great-free-alternatives-to-autocad