First you need to download the latest Raspberry Pi Raspbian OS from here. Get whichever version you want but I like to get the desktop version so I can VNC into it as it makes it easier.
If you have never done this connect your Raspberry Pi to a monitor, keyboard and mouse. If you are an old salt with Linux then just run headless and SSH into your Pi.
Once you download that file UNZIP IT. UNZIP IT. The file will end with an extension of .img
Do NOT try to burn the zip file to your SD card. UNZIP IT AND GET THE .IMG FILE.