You should have your mouse's home (cage, tank, whatever) perfectly ready before getting the mouse to live there. At the very latest, you should buy the home when you buy the mouse. Consider very carefully what type of home will best suit your mouse and your needs. Getting a cage hastily may end up costing you more in the long run, if you find that what you bought is unsuitable for your mouse and you are forced to buy something else.
There are many kinds of mouse homes available. It's up to you to choose the most suitable one. If you are able to create your own mouse house, it should fit the following requirements:
It should be safe for the mouse. There must not be anything in the house that mice could hurt themselves on.