I remember last year I was going to start playing around with Atlassian JIRA API because of the massive installation my employer has (hosted internally, today's version: JIRA v6.3.11) and the experience was like an uphill battle.

If I want to get started again, where would you suggest me to look? I believe one should start with basic JIRA knowledge to understand the fundamental of the software/system/service before they continue to tackle the API (I believe most people, except Managers, use JIRA as told without knowing some fundamental idea behind it).