Go to https://smashballoon.com/instagram-feed/find-instagram-user-id/ and enter your Instagram username to get your User ID. Copy and save this ID somewhere.

Go to http://instagram.pixelunion.net/ and click Generate Access Token. You will then be asked to login to your instagram account (if you aren't already) and approve access. Once granted, you will be redirected page to a page that contains the new access token. Copy and save this value.
These are the two values (User ID and access token) that you will need in order to make API calls from various modules/widgets. Note that the User ID will always remain the same, but the access token may eventually expire (after a long period of time), in which case it you can follow step #2 to generate a new one.