There are a lot of different recipes out there. Which one you use will depend on your goals. Are you trying to keep costs low? Quality high? Specialized gardening needs?
Here's a place to start:
1 part soil (if you have good soil you can use your own, get it from someone who has good soil or buy some)
1 part vermiculite (I found a really huge bag for cheap)
1 part peat moss (some folks object to the environmental aspects of peat moss; coconut coir (expensive!), leaf mold (free!) and manure (free!) are substitutes but please note that the manure acts as a fertilizer)
You'll want to research your specific plants, experiment and adjust this recipe as needed.
Here's a nice article:
https://extension.psu.edu/home...
