Whatever your motivation is for starting seeds indoors, the process can be fun and simple. When you understand what factors influence a seed you'll be able to create a formula for success, and then repeat it again and again.
First, it helps to understand what a seed is. Then, a look at what it responds to will help you understand what it takes to turn a seed into a plant. A seed is the embryo of a plant. It usually comes packaged with some food and is enclosed in a protective coat. It is actually a dynamic little (or not so little) system of interactions that functions to preserve the genetic potential of its parents until it is time to grow.
Seeds respond to water, light (or absence of light), and temperature. They do this within their physical environment. In the case of starting seeds indoors, this is created by the container and soil you sow them in. Making good choices for soil and containers will help you grow a strong plant, able to live a healthy and productive life.
There are lots of choices when it comes to picking seed-starting containers. You can find all sizes and shapes of ready-made seed-starting supplies at your local independent garden center that are specifically made for this purpose. Almost any household container can be re-purposed as a seed-starting container. Once you know what makes a good seed-starting container, you are only limited by your creativity. Containers should be clean. If you buy new, you can assume they're clean. If you choose to re-use or create your own containers, they should be sanitized to remove any possible pathogens. Soaking them in a 1:9 bleach:water solution will achieve this. Besides being clean, there are other considerations that make a good seed-starting container. Drainage is essential. Any container you choose should have a way for excess water to drain away from your soil. The way you will transplant your seedlings will also inform your decision-making. If you will transplant your seedlings as single plants at a distance from each other then individual pots or divided trays will be the best choice. If you will plant your seedlings in clumps or close groupings, then a broad or wide, shallow container could also be suitable. The size of the seed you sow, the length of time to transplant, and the size of the resulting seedling will all influence the size of the initial container you use. Large seeds (i.e. squash, castor bean, four o'clocks), plants that grow indoors long before transplant (i.e. artichokes, celery, perennial flowers), and quick-growing plants (i.e. tomatoes, lettuce, pumpkin) all denote the need for a larger initial container.
The soil that you choose to start your seeds in is as, or more, important than the container. Use a high quality seed-starting mix. This is often a case of, "you get what you pay for," and is not the place to sacrifice quality. Buy a mixture that is specifically labeled for seed starting. Do not use soil from outside. It can harbor microorganisms and pathogens that, when taken out of the balance of nature, can harm or kill your seeds. High quality potting soil is usually made from fine-ground peat moss and often contains vermiculite or something else to increase water retention. This mixture allows for a moist, not soggy, environment with the right mix of air and water to promote germination.