@vedant.kulkarni in order to use atlaskit packages, you should have experience in ReactJS development. If not, then you need to get such experience or use AUI library as an alternative way to build your Atlassian interface.
As for the Atlassian Connect application, ways to implement your custom addon depends on the language and framework that you’re using. You can look through the list of frameworks and tools here. https://developer.atlassian.com/cloud/jira/platform/frameworks-and-tools/
To use framework is the fastest and easiest ways to start building your own Atlassian Cloud addon. So you can start with a framework, its structure, and docs for developers. Also, you can find a way to achieve the same results without pre-configured frameworks, but if so you need to read and understand the requirements from documentation.
I believe we’ll see your first Cloud addon soon. Welcome to the Atlassian Cloud Developers Community.