Enclosures / Housing
There are several different types of substrate available, but I personally have been successful using a combination of 50% peat moss and 50% vermaculite in most of my enclosures. The vermaculite seems to "smooth out" the peat moss and allows for adequate moisture retention. Both peat moss and vermaculite can be purchased a home repair store such as Lowe's or Home Depot.
One of the most important parts of keeping a tarantula is making sure that it stays hydrated. The amount of moisture needed varies between species, but I've found that many will do well with an open water dish (no sponge) that is kept full. A water dish can be the type purchased at a pet store or simply some type of lid from a food container.
A full water dish will will help to ensure that your tarantula has adequate humidity. For many "beginner" species - G. rosea, B. smithi, etc. a full water dish will provide all the humidity that they need.
Hide / Retreat
Many terrestrial species will benefit from some type of hide in which they can use to feel safe. I've found that the easiest way to make a retreat is to purchase a ceramic or plastic flowerpot, place it on its side, and bury it partially into the substrate.
Arboreal species tend to make web retreats, but will need something to attach their webbing to. Cork bark, which can be purchased at many pet stores, is a nice addition to any arboreal tank.
Most species can be comfortably kept at room temperature (approx. 72 degrees F). If your home typically stays below room temperature, you may want to consider adding a heat mat to your tarantula's container. Heat mats should be placed on the side or back of the container.
Adequate containers for housing can range from plastic tubs purchased at Wal-Mart to aquariums purchased at a pet store. I've found that 2.5 gallon and 5 gallon aquariums generally work well for most species. The size of the container should be appropriate for the size of the spider. A small spiderling will need a small container or vial so that it can easily find its food while a large adult tarantula such as T. blondi would be best suited for a 10 gallon aquarium or larger. Below are some examples of simple housing ideas.