I play DotA some.
Download the map here. http://www.getdota.com/
Then use this server to play it online for free. Garena.com
I reccommend trying to get good with practice and know what your doing before u play on Garena. Their pretty hardcore. Also DotA=WC3 RPG on the server.
Heres a site for some tips that help out alot: http://www.dotastrategy.com/

Also the Campaign pretty fun alone, if you haven't played it.