Best site is http://mapa2.buenosaires.gob.ar/ If you click on "como llegar" and put your starting & end points, you can see all the ways to get there (bus, suway, train, etc).
The Guia T is your best friend where local busses are concerned, but it's a bit hard to read if that's your starting point and you don't know the bus routes. Otherwise, my favorite is comoviajo.com, as someone else recommended above. It's pretty great (if it's running!) as long as you know your start place and general destination. Then travel with your Guia-T in your pocket and you're golden.