To create your personal API token on a CircleCI server instance, you can go to "User Settings" and then to "Personal API Tokens".

From there, you just have to click on "Create New Token" button, and give it a name. Copy the API token displayed on the screen.

PLEASE NOTE: You will not be able to retrieve the value of this token again after creation, so please remember to copy it now, and if you are able, to store the value in a secure place.