Right, you need to have a House License which you get from the DVLA, once you got that find your perfect house, go to the door and hit Windows key, if it's not already owned you can purchase it (the money comes straight out of your bank so no cash in hand required). Once you own it press U to open and close the door, then go inside and hit Windows Key again and it will display a menu which has Rent, Upgrades, Storage ETC. You no longer need Storage boxes to store stuff, however, if you want to store magazines for weapons then you will need to upgrade it. To put items in your house simply select the storage tab and drag items across into the houses' inventory.
Hope that helps, any other Questions feel free to ask!
- Louie Austino