Seeds are like people: they grow in different ways. Generally speaking, the smaller the seed, the closer to the surface it has to be. Larger, harder seeds may need help to germinate. Most seeds need light and some prefer near dark conditions to germinate. Some seeds germinate in a few days, most in a week or so, and some take months to germinate. Orchid seeds are an example of a long germination period. Lettuce is an example of a seed that quickly germinates. I have found that misting seeds is a really good approach in the beginning. Obviously, one can over-water seeds or allow the seeds to dry out and stop the germination process. As a general rule, keep the seed soil always a little moist - mist, don't water soak seeds or little sprouts because they are too delicate for that pressure.