Hello Devarsh Shah,

You are free to assign any unassigned issue to yourself. Though, I is strongly advised to discuss with the team in the chat before starting working on a task.

For instance, Cristal is really in not in a state where we can easily onboard new contributors currently. So, I'd advise not to start with it.
See also https://dev.xwiki.org/xwiki/bin/view/Onboarding/