The basic soil-less potting mix found on big box shelves is composed mainly of Sphagnum Peat Moss some perlite and often contains fertilizer. It can become very expensive particularly for those gardening in containers.
Gardener's can mix their own at lower cost in order to grow more plants.
Here is a widely recommended seed and potting mix that can be adjusted to one's particular climate and plant needs.
Basic Mix with Compost
2 parts well composted manure and other compost. Finely screened mushroom compost is an excellent option.
2 parts Sphagnum Peat Moss or Coconut Coir (Either retain significant moisture. In cooler slower drying conditions it may be wise to lessen the amount of either in the mix.)
1 part Perlite
1 part Vermiculite
Perlite and vermiculite are both good at retaining water, but vermiculite acts more like a sponge, holding much more water than perlite and offering less aeration for the plant roots. Perlite retains water because of its large surface area with nooks and crannies available for water storage. Because it is porous it allows excess water to drain more readily than vermiculite and improves soil aeration. In cooler slower drying conditions it may prove wise to lessen the amount of vermiculite and add more perlite and/or sharp sand in the mix. Sharp sand (builders sand) maintains looseness of the mix and aids drainage.
Basic Mix with the Addition of Nutrients
Add ½ cup each per every 8 gallons of mix:
½ cup Bone Meal (Phosphorous)
½ cup Dolomitic Limestone (raises soil PH and provides calcium and magnesium)
½ cup Blood Meal or Soybean Meal or Dried Kelp Powder (Nitrogen)
The single greatest cause of plant failure is over watering and the resulting growth of bacteria and fungus. Select seed trays that can be watered from the bottom which prevents disturbing the seeds. Be sure to remove excess water from the watering tray once the soil is saturated.
Remember to sprinkle cinnamon on the surface of the seed tray after planting to deter gnats and kill fungal spores.