If you have just bought the sd card.
Before it can be used by windows or any os such as Android ect.
It must be formatted by using the settings options of your android ect.
With windows, mac simply right click on the drive and format it.
You cannot read or access a SD card till you format it if you just bought it and put it in a phone or a system it has to be formatted so the os can read it and lay a file system format down it can use and see.
Once done the card will be read and mounted as a storage device showing its free space, how much is free.