NeOCidE you can do it from the game but then it is called watching replays. Look for a sort of video camera icon in the menus. When u watch streams go to sites like www.gosugamers.net or www.teamliquid.net and look them up. You can also google for big leagues like IPL, GSL, NASL, MLG etc etc. But the two sites linked above is a good start.
Oh another good source for SC2 games is Youtube search for StarCraft 2 Vods (video on demand).