It is essential that the soil you use be sterile. One of the great bane's of seed growing is a fungus commonly known as Damping-off Disease. The spores of this fungi are ever present in soil and the conditions of indoor gardening seem particularly advantageous to its propagation. It can wipe out hours of hard work in a matter of days. To avoid this scourge you must use a sterile soil or, even better, another sterile medium.
Some gardeners like to use their own soil.
This involves sifting out any larger clumps and stones and then sterilizing it in an oven. This is a lot of work and the results are probably no better than using a commercially available starting medium. This should be clearly indicated on the packaging. After it has been thoroughly moistened, the medium can be placed in flats, peat pots, Dixie cups, egg cartons, etc. The size of the container will depend on what type of plant you're growing, and often experience is the best gauge.