ProMix. If you can find it where you live. It's made from peat moss, vermiculite, perlite and limestone. The limestone acts as a pH neutralizer that keeps levels in the ideal range for most potted plants (including cannabis) so you don't have to worry about pH levels. The vermiculite and perlite give it really good moisture retention. It's also really cheap where I live. Very popular product with bikers.
Failing that, buy some good quality potting soil and stay on top of the pH and I think you should be fine. I know some people will mix in shit like sand, more perlite, and plant-food mixes with their potting soil, but if you're just starting out like me it's probably better to start simple right?
Miracle Gro is one of those things some people swear off, and some people have always been able to use successfully. I think that the MG with time released nutrients in it is really bad for cannabis plants.