If you are completely new to mushroom growing, I recommend that you skim through a copy of Growing Gourmet and Medicinal Mushrooms by Paul Stamets. Paul's book will give you a good overview of growing mushrooms of all kinds. It has plenty of pictures, and there are descriptions of most of the mushroom species that can be reliably cultivated. It is also a valuable reference volume, and I use it regularly. Starting with a tissue culture assures that you have a mushroom strain genetically identical to the one you want.
Some growers start with spores, which are more like seeds.
Spores may or may not give you a mushroom strain with the fruiting properties of the parent. Since spores cannot be grown in the presence of hydrogen peroxide, I always work with a tissue culture of mycelium. If you buy a mushroom kit, you are generally starting at step four. The commercial mushroom grower has already completed the earlier steps for you, and provided you with the mushroom culture ready to form mushrooms. You provide a proper environment, usually cool and moist.