Before you start, you (obviously) should have a microSD card and a computer with an SD card reader. 

SD card

SD card reader

Step 1:

Now you should download the latest version of Raspberry pi from its official website here. 

Step 2:

Note: the Raspbian with Raspberry Pi Desktop image contained in the ZIP archive is over 4GB in size and uses the ZIP64 format. To uncompress the archive, a unzip tool that supports ZIP64 is required. The following zip tools support ZIP64:

• 7-Zip (Windows)
• The Unarchiver (Mac)
• Unzip (Linux)

Step 3:

Now you should download software which allows you to write on your SD card.

• Download balenaEtcher and install it.

Step 4:

Writing an image to the SD card

• Connect an SD card reader with the SD card inside.
• Open balenaEtcher and select from your hard drive the Raspberry Pi .img or .zip file you wish to write to the SD card.
• Select the SD card you wish to write your image to.

In this video, you can see the following steps.