Plastic sheds are generally the cheapest options, and can be purchased in a number of different sizes, colors and styles. The benefits of plastic storage sheds is that they are low maintenance and waterproof. They won't rot like wood sheds,rust like metal sheds, and assembly is generally quick and easy. Some drawbacks to plastic sheds is that while they are cheaper, they are not as durable as other types; they are also incredibly lightweight, and can be blown over in windy areas.
Now that you know how many boxes you need, you need to know how much space you have for them. Remember that the whole idea of using storage boxes is to be able to clear things away, and not to have a room filled with these boxes. You must be able to hide these boxes, away from exposed areas. You should also get the measurements of where you plan to put these boxes. Measure how much space you got under the bed, inside your wardrobe or above it. You wouldn't want to purchase boxes and still end up living it on the hallway because it won't fit anywhere. So remember to take measurements, write them down and bring it with you when you go shopping.