First of all, you need to download your Kali Linux iso file just head over to the below link and select your preferred version of your Kali Linux.
It is recommended to download the standard version of Kali Linux as it has all the key tools for your pentesting. In case you are going to use Kali Linux for only some particular task, you can go with the Lite version.
For making the iso into bootable you need a simple application named “RUFUS” which simplifies the entire process. Just head over to this link and download Rufus and install it on your computer.
Now Open the downloaded Rufus application and plug your USB pen drive to your computer. [Do note all your data in the pen drive will be deleted. So don’t forget to take backup of those data]
Now select your pen drive and iso which you have downloaded recently. Then, press the start button and have a coffee. After this process reaches 100 % kindly unplug your pen drive from your computer.