This question has got to be the most commonly asked question. There are two easy mixes, both work well. Some people mix mericle grow potting soil (3 month feeding) with peat moss and perlite. This works well because it tends to hold water well and you wont need to add any nutrients for the first 3 months of vegetative growth. Another mix leaves you to decide what nutrients you want to put in it. I use this myself. Its a mixture of Pro-mix (peat moss) Perlite, Vermiculite, and a dash of gypsum to buffer the PH. You can add worm castings to this, as much as a 1/4 casting to 3/4 soil mix. Remember, you dont need any nutrients at all for the first 2 months, but the worm castings and mericle grow/peat moss mixes will give you a good head start while avoiding nute burn.