Welcome to the Atlassian developer community, @vibhugupta!
I recommend starting with our developer guide to get familiar with the whole Atlassian landscape. Then, take a look at the Jira Cloud platform REST APIs to understand the APIs in general. With that context, you should be ready to get an issue with GET /rest/api/3/issue/{issueIdOrKey}.
When you have more questions, I recommend looking over how to ask a question so the community will be able to help you out.