The advantage of a greenhouse is that it allows someone to grow plants throughout the year, and virtually ignore the normal cycles. That is, by tricking the plants into thinking that it is spring or summer, plants can be grown throughout the year rather than just a relatively few months.
While there has been some debate on the best way to start seeds in a greenhouse, it usually comes down to some variation of heat and humidity; by manipulating those a grower can encourage his plants to germinate and sprout virtually any time that he wants. It is just a matter of finding the optimal combination for a group of plants.
It should be noted that this is not as difficult as it sounds. Seeds are programmed to start sprouting when they find the optimal conditions and when conditions are right for growth. This means that things break down to two different sets of conditions.
The first is that the seed must fall in the right type of soil for the seed, usually something moist with lots of nutrients. A grower can encourage this by making sure that his soil is ready to go by fertilizing it and keeping it moist. The most common mistake is that some growers keep the soil too moist, and so seeds rot before they have a chance to sprout.
Keep in mind that the other factors need to be checked if the seeds are having issue sprouting; the most common are temperature and acidity. Most plants require that the ambient temperature be somewhat warm; this is actually the point of the greenhouse. If the greenhouse is too cold then it is necessary to move some heaters into the greenhouse in order to stimulate growth.
It may be necessary to check pH levels of the soil, as some plants require slightly acidic soil to grow while others require a more alkaline soil. As it is easy enough to adjust the soil with the right kind of fertilizer and other treatments, make sure that the soil is of the right pH factor for the plants in question.
By the same token, it may be necessary to adjust the lights. Some plants actually fare better with a slight bit of shade than they do in bright sunlight, but it is easy enough to dim the lights as necessary. It is even possible to mix plants with different sunlight requirements by having the bright lights in one area and almost none outside of that area; the rest of the plants can use the ambient light as needed.
Keep in mind that ultraviolet lights may be necessary in order to simulate natural sunlight if the greenhouse is in a place that is unusually dark or that have too much rain normally.
All of this combines to create the best way to start seeds in a greenhouse. It is just a matter of determining the best conditions for your plants and giving them what they want. Do that, and you will have some very fast growing healthy plants ready for the table or market in short order.