I like a rich organic mix, I start with a base soil then add stuff like worm castings, bat guano, bone meal, phosphate, lime etc, lots of perlite and a few hydro balls for drainage.
Theres lots of good mixes on google.
Or just do a lazy cheap one and use warehouse or bunnings potting mix use lots of perlite again for drainage especially important when using those kind of wet mixes. If you can water it often that is, obviously if you cant water daily you will want vermiculite and water crystals added.
Also with autos I like to top them once just above the second node to get four main colas, only top once as you are limited veg times with autos so you want to maximise it as much as possible, I find I get more from autos when i top them, rather then growing out naturally, even though the topping will stunt it but only lose a day growth If that.
I like growing autos.