The basic object in seed starting media is good drainage and moisture retention. You can use a lot of things; I've used pure vermiculite to good effect, but once seedlings have germinated you have to feed with every watering since it contains no nutrients. This is fine for fast growing annuals and veggies, but doesn't work well for perennials and plants who will remain in the seed pot for some time.
Do not use soil from the garden. It won't drain well and it's full of weed seeds (as is most compost) and assorted pathogens. Fine for plants growing outdoors, but not in controlled seeding environments. My first foray into planting seeds started with just that mistake. I coddled the nicest lot of smartweed you ever did see. My poor flower seeds didn't have a chance.
My mix of choice, at this time, is the peat based Pro-Mix which contains perlite and a small amount of fertilizer. Sometimes I add extra perlite or grit, depending on what the ultimate plant might prefer and the mood I'm in. This comes in bales, dry as a bone. Peat products are hard to wet. I fill a bucket and add really hot water, stirring and squishing it up until it's thoroughly damp - not soggy.
I use granite grit to top the pots off. I buy this from the local feed and grain store, where it's sold as poultry starter grit. Just make sure that it's pure, plain grit, without any added calcium or salts. This keeps the soil from washing and helps maintain even moisture for the seedlings. It also changes color when it dries out - from light gray when wet to white - providing a notice that the pots need a drink.
Fill your pots to the bottom of the rim line and gently firm the mix (you want it about a half inch (1.27 cm) from the top when firmed). Sow your seeds evenly (about a half inch [1.27 cm] apart - more if they are large) on top of the mix and press into the mix gently if they are big enough to see without a microscope. Sprinkle up to a quarter inch (0.63 cm)of grit on top of the pots and water gently until water runs out the drain holes.
If you're working with really fine, dust-like seed, top the mix with grit and then sprinkle the seed over the grit and water it in.
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