While readily available premixed in most garden centers, most commercial sowing media here is approximately equal parts peat (you could substitute coir) and vermiculite, sometimes with some perlite added. If starting plants that require alkaline soil, a bit of lime may be added.

You'll want to moisten it before preparing your pots - the correct amount of moisture and it will feel about as damp as a well wrung out sponge, not soggy.

Seedlings will normally be moved out of this mix and into their own pots when they have developed their second set of true leaves, but garden soil still is not recommended for potted plants, too heavy in almost every case. You can buy a ready made mix if you don't want to purchase ingredients to make your own and improve it by adding more vermiculite, peat, perlite, or horticultural grit (similar to parakeet gravel).