Your seed starting mix should be a finely ground mixture of peat moss, perlite and vermiculite.
Peat moss: is naturally spongy therefore allows for increased water retention.
Perlite: decreases water holding capacity which results in better drainage. It also increases air porosity
Vermiculite: increases air porosity
This ensures that the seedling don't have any huge particles to try and grow around while maintaining good aeration of the media with lots of space for water retention. It is essential your seedling mix be sterile. You do not want weed seeds, bacteria or other things attacking your seedlings as they grow.
For more detailed information on each of these components and their benefits, PT Horticulture has great videos by Horticulture Specialist Troy Buechel. They are 3-5 minutes long each and full of great information. Here are the links to the videos:
Perlite: PRO-MIX Growing Media Components Explained: Perlite| PRO-MIX Greenhouse Growing (pthorticulture.com)
Vermiculite: PRO-MIX Growing Media Components Explained: Vermiculite| PRO-MIX Greenhouse Growing (pthorticulture.com)
We hope you enjoy the videos and get as excited about growing as we do!!