@shivamluthra: Thanks for your questions. Aplogies for not being a bit more timely in getting back to you here. In general, asking questions quickly, before you get demotivated, is the best way to get unstuck. Don't worry, we do not expect that newcomers to our community and our software will magically know how to do everything. Asking questions frequently and quickly is the sign of a good learner. You've done the right thing by coming here to ask.
In answer to your first question, we use the issue tracker in Products.CMFPlone as the central reporting place for issues related to Plone, but the code related to any bug will most likely be in one of the other packages in the repository. Asking questions on individual bugs will help you to find the right place to look for a fix. You can use the github feature of using the
@ sign in front of a user's username to address a question to a particular person. In general, the user who reported an issue would be a good first person to ask such questions.
For regression bugs, where you want to compare various versions of Plone, each release has a corresponding tag so you can check out a given version of Plone using git. It might be better, though, to use the installer for a given version of Plone to compare. That way you are assured you will get a fully working system without needing to spend time changing buildout configuration. You can install multiple versions of Plone on the same machine without them interfering with each-other. On our plone.org website you can find recent versions of the installers as well as a link to many many older releases.
Finally, to answer your third question, there is a page explaining most of the names for the various tags in the issue tracker written in our documentation on contributing to Plone. I think for a person knowing front-end skills, you'll want to check out issues tagged with
99 tags: UX .... There are a number of tags for Accessibility, our visual editor (TinyMCE), Integrator/Themer and Site Administrator. Being UX tags, most will have something to do with the visual interactions in Plone, and would likely contain work requiring front-end skills.
Do not be concerned if you can't just "fix it right up" without some help. The point of this program is to help you learn how to contribute, and that includes helping you to fix your first issue. We enjoy working with new contributors and helping you to succeed, since your success is ours as well.
Again, thanks for bringing your questions here, and please keep them coming!