So you're going to develop something for JIRA Server (internally hosted software). You'll be using Plugins2*

1. Start here https://developer.atlassian.com/jiradev/getting-started/gett...

2. Install the SDK (it has some pre-reqs) and build Hello World https://developer.atlassian.com/docs/getting-started/set-up-...

3. Bone up on Plugins2 a bit more https://developer.atlassian.com/jiradev/jira-platform/buildi...

4. API Reference https://developer.atlassian.com/jiradev/jira-apis (I recommend the REST APIs + Webhooks wherever possible).

I'm really keen to talk with you about your experience going through that - there's a lot of history and we're aware we need to clean up the journey.

_* If you were developing for JIRA Cloud you'd be using Atlassian Connect.