How can I start application development for Power BI?

I have few questions regarding application development in Power BI.

I want to prepare an application which takes data from third party application like JIRA,GIT or VSTS.

I want to publish this application as publically available in Microsoft App source. When end user click on Get IT Now from Microsoft App source they need to provide application URL , Username and Password.

I don't want to use Power BI premium licence for that.

I have few questions regarding this development.

1. How can I start with first? Do I need to prepare Power BI report by getting data from third party application first or I need to focus on Authentication process and loading a data first?
2. Is there any example available for similar kind of development?
3. How can I test this using App source?

Your answers are appriciated.