This is probably a common issue for people who wants to deploy serveral repository on one VPS or VM.
This is not going to be rocket science.
The first step is to generate your first ssh key, for this type the following command line in your terminal:
ssh-keygen -t rsa -b 4096 -C "firstname.lastname@example.org"
When the command CLI is asking you if you want to use a
passphrase you might want to press
ENTER in order to don't have to type it everytime you will want to pull your repository.
Now copy the content of your public key and add it to Github in your first repository (Settings > Deploy keys). For example on debian it might be: