Important note: Never share your private key with anyone, you rarely even have to touch this key so keep it safe and only for your own eyes.
Generating a SSH key is not that hard, open up a terminal and type the following:
ssh-keygen it will now ask you some questions on where to store the SSH key, remember that path or customize it to your needs. You will also get a question on the passphrase, you can either leave this empty or type something random.
Whenever its created (and you can see this by the 'randomart image' that is being displayed after creation) you can copy your key:
The above code will get your public key if you have not modified the path that was asked in previous questions, you will now be displayed with the SSH key in your terminal. You could also copy this straight away with:
cat ~/.ssh/id_rsa.pub | pbcopy
This will ensure the key is copied to your clipboard straight away so you can enter the key into your Ploi profile, or server.