The first line of defense is, of course, the garden catalogs. This is the time to pour over the pictures and imagine a garden full of every delightful plant possible. I find myself dwelling over Johnny's and wondering why I can't grow exotic varieties of vegetables that I've never even eaten. My particular obsession is with lettuces - nothing more beautiful than lettuces like blooms on the soil, especially to my winter eye. Seed catalogs often tempt one to over-order but there are also stalwarts that simply must be obtained every year. I have a number of favorites that my garden would miss if I didn't order them.
But the consequence is that the seed packets arrive in the mail, and many of them need to be started indoors. This is a benefit. It is an opportunity to work with living things and see the green of chlorophyll in spite of the snow cover.
The techniques of seed starting can be very simple or extremely complex. Most garden plants have been selected over hundreds of years to be started easily from seed. Wild or recently domesticated plants often have dormancy mechanisms that need to be overcome before the seeds will germinate. The survival value of this is obvious. Seedlings may be killed in one year's bad weather and others can germinate in a different year to flower and set more seed. Here is a discussion of factors that inhibit germination. Here is a master list of conditions for seed germination in many wild species.
Even easy garden plants have requirements. Seed germination requires several environmental triggers to be present, and the combination of these varies somewhat from species to species. First, water must be present. Even dead seeds will soak up water (imbibition) and it is the first step in germination. Light and temperature are the other two triggers. The right combination, and sometimes the right sequence, will cause a complex series of hormonal changes in the seed that allow the embryo to grow and use the stored food reserves. Here is a scholarly review of the physiology of seed germination.
For the great majority of garden plants, higher temperatures (around 70° F) are best for germination. This can be supplied by "seedling heat mats" for sale in many garden stores and catalogs. There are also many light fixtures for growing plants (fluorescent lights are best) that can be fancy or simple. I use shop lights propped up on blocks. It is necessary to keep humidity high while germination occurs - this can be with plastic domes, plastic bags, or even transparent sweater boxes. A timer should be used to turn lights on and off. I use a 14-hour day length.
Seedlings are easily killed by a couple of common fungi. This is called "damping off". For that reason, seedlings should be started in sterile artificial medium, typically sphagnum peat that has been finely milled. The germination medium I use also has vermiculite added to keep it from clumping.The medium should be kept damp, but not flooded. I use a plastic spray bottle to keep dampness at the right level.
In the cell packs, the plants can grow under the lights, and later be moved outside. I use a cold frame to temper them until it is consistently warm enough to plant. But they'll last in these cell packs until June or later.
ADDENDUM: A question left unanswered here is, "When should you start the seeds?" This depends on your location and the first frost-free date for your area. In Southern Lower Michigan, it is approximately May 15, so vegetable seeds should be started with that in mind. Johnny's Select Seeds has a handy seed-starting calculator for that decision. Flower seed starting is variable: some need to be started as early as January in order to bloom in a reasonable time after they are planted out. Petunias take a very long time to become planting size, for example. Hot-weather annuals like zinnias and marigolds will need to be held until the frost-free date, so that should be taken into account. Some others, like pansies and violas, can be planted earlier so should be started earlier. Perennial plants often grow slowly so benefit from an early start. The seedlings shown above were all planted this year (2014). A cold frame is useful to temper seedlings in the couple of weeks before the frost-free date.