If you can find them, it's best to keep a couple simple ingredients on hand to make 2 different soil mixes, one for seedlings and one for more developed plants. Others add more "active" ingredients, but I've always just made a base mixture of potting soil, perlite, and vermiculite. The soil and perlite will be available at most any Home Depot or garden store. The vermiculite is a little harder to come by, but I found it's properties in soil to be worth going out of my way. My favorite potting soil brand will likely be available at a store that carries vermiculite - Foxfarm's Ocean Forest. It's nice and rich, the plants love it.
For seedlings, Mix soil
, perlite, and vermiculite at 2:1:1. This soil will be fluffier, and more amenable to early rooting. For more developed plants, mix at 3:1:1.
Do some reading and good luck!