ssh-keygen -t rsa
The default save location of the key pair (public and private key) will be shown. It should always be ~/.ssh (/home/yourusername/.ssh).
After you confirmed the location the key pair is stored you are asked to enter a passphrase. That is your password for every system your public key of the pair is stored.
At the time of writing the default length of the key is 2048 bit.
Source: How To Set Up SSH Keys