Earth easy gives some great advice on building your soil for planters and raised beds.
He gives tips on how to begin by adding to the soil to make it more fertile and then goes on to tell us what we should do to keep it that way.
If you have a garden, whatever you grow in it will pull nutrients out of the soil so you need to always add nutrients back to keep it fertile.
Great information in this article for beginning gardeners and seasoned gardeners as well.