You will need a MicroSD card for you RaspberryPi with the Operating system on it. You can purchase a MicroSD card from various commercial sites with the operating system already installed but it is not difficult to download and install it yourself providing that your home computer has an SD reader.
To install the operating system yourself, download the latest Raspbian file from:
Unzip the file to get the image for the SD card.
You will need an imaging software for the SD card:
Win32DiskImager is the usual choice for Windows PC and can be downloaded from:
Download Win32DiskImager and unzip the file. You can run the exe file even from a USB drive.
Note: Be careful when you use this utility as you could format the wrong drive on your computer.
If the SD card has been used before you will have to reformat the card before you do anything else.
Select the image file you extracted and select the drive letter of the SD card in the device box.
Make sure that you have selected the correct drive letter.
Click Write and wait for the image to be written to the SD card.
Exit the imaging software, eject the SD card and insert into your Raspberry Pi.