Step 2: Choose the material
You have many options to chose from. Things you need to consider are:
How much material you need to buy?
How many modules are you building?
Can you cut it yourself or do you need a machine?
What material suits your apartment?
Do you want to paint it after?
Does it need to be water resistant?
What will it hold?
Is it too delicate for books?
How much weight can it resist?
Some options that could be used are:
$ Cardboard: Decorative shelves mostly
$$ Foam-cord: Very decorative, maybe flowers or light stuff
$$$ Wood: Great for books and heavier stuff
$$$$ Plexiglas or acrylic: very modern and minimalist but you need a laser cutter