Hi @Shraddha and welcome to this community 
Shamelessly quoting @julieheard :
Here are a few links that hopefully will be useful to you:
- Contribute code - How to contribute to Jenkins
- jenkins/CONTRIBUTING.md at 03c380f6c451a1a05e3b0a8471f3aa06a24c3baa · > jenkinsci/jenkins · GitHub - This is more detail on how to contribute, a bit more technical than the previous link
So I would read through those websites, then take a look at the issues Jira. When you have picked a task to work on, make a PR and wait for feedback/approvals.
Good luck and reach out if you get stuck/have more questions
