What is considered the "sweet spot" will vary from amp to amp, player to player, engineer to engineer, song to song, and even tone to tone. Search these forums for more advice/recommendations/discussion than you can possibly imagine. FWIW, my general favorite that I've read that a lot of folks like as well is about 1 foot away from the amp and about 1/3 of the way from the center to the edge of the speaker cone. The sound varies by a surprising amount as you move across the face of the speaker toward or away from the center line, and the closer you get to the amp the more sensitive the left to right placement becomes. I also like the sound better with the amp up off the floor (I keep mine stacked on another amp that I don't use anymore) and away from the wall a bit (6-8"). As others have said, ya kinda have to try it different ways 'til you find what you like, but those are the basic parameters I usually(almost always) work with.
J