The 2012 Oscars award show, formally known as the Academy Awards, will begin at 8:30 p.m. Eastern Standard Time, or 5:30 p.m. local time. The live broadcast doesn't have a definite end time, but the show almost always wraps up before midnight.
ABC will be the exclusive live broadcaster of the awards show, and which channel ABC is on depends on your cable or satellite provider, as well as your location. If you subscribe to Comcast, Time Warner Cable, Dish Network, or DirecTV, you can use their online channel finders to figure out where to tune in.
The Red Carpet shows will start earlier, with ABC's official show starting at 7:00 p.m. EST. MTV's red carpet coverage will start at 5:30, and E! will start at the ridiculously early hour of 1:30 p.m. EST, and will be hosted by Giuliana Rancic and Ryan Seacrest.
ABC will unfortunately not provide any online stream of the awards show, so you'll need to be by a TV to tune in (well, at least legally).