The primary text for "The Storehouse Principle" is found in Deuteronomy 28:8, "The Lord will command the blessing upon you in your storehouses and in all that you put your hand to..." Only two other times in the Bible does God "command" a blessing. In this case, the blessing is commanded upon our storehouses. So what exactly is a storehouse?
King David created one in order to gather supplies for the building of the temple (1 Chronicles 27:25). Nehemiah had storehouses for the rebuilding of Jerusalem (Nehemiah 7:70-72; 13:12-13). Hezekiah had storehouses (2 Chronicles 31:6-10; 32:27-30). Of course, Joseph was made famous for his storehouses (Genesis 41). A storehouse is simply a place of savings--a place where you put your surplus. It's like a savings account.
Here's a couple of things that we need to grasp in order to have a complete biblical understanding of finances. First of all, God has already provided more than we need. He is the God of more than enough! He has never done anything without providing more than enough. There are numerous examples throughout scripture. When the Israelites were picking up manna in the desert--there was always more than enough. When Jesus fed the 5000--everyone ate until they were full and then 12 baskets of leftovers were collected. God is not limited in His resources. The second thing to understand about our finances is that we tend to either hoard or spend if we believe that resources are limited. People, however, with an abundance mentality are able to wait and save. You and I will be able to set up a storehouse only if we come to believe that there are plenty of resources. A third thing to understand about your finances is that God blesses us so that we can bless others. He doesn't give to you so that you can just selfishly use His blessings just for your own purposes. We are stewards (managers) of all that God sends our way. God owns everything and He allows us to simply manage a portion of His resources.
How do you begin a storehouse? A storehouse begins with anything that we set aside to save. If you find a penny--pick it up and put it in your storehouse. If you get a extra money--out it in the storehouse. You can have a garage sale. You can sell stuff that you no longer need on Craig's List or something like it. You can dedicate a percentage or an amount of each paycheck to go into your storehouse. When you get enough money collected you might want to take it to the bank and open a "Storehouse Account". Don't worry about starting small--just start! Then once you've begun, ask God to "command a blessing" upon your storehouse (Deut. 28:8).
When can you spend your storehouse? Only when God says that you can. Consider it His money. Never use it unless you are absolutely sure that God has asked you to.
Here are some New Testament passages that might provide a bit more insight: Luke 16:10-13; Matthew 25:1-13; Luke 19. You might want to look these up and read them and ask the Spirit to give you some application in regards to starting a storehouse.
Now, go and get started!