I'm working on an issue in the Bitcoin Core. This is my first issue and so I have some questions. Is it good practice to open a pull request and ask reviewers of the pull request to help me or I should ask them elsewhere (such as IRC)? I think PR page on github is a good place to ask my questions. Note that I currently have not done anything worth mentioning.
You can open a pull request in draft if it isn't ready to be reviewed yet but it should at least be a work in progress rather than its current state ("not done anything worth mentioning"). Reviewers are unlikely to review it if it is in draft but adding to the open pull requests without it at least being a work in progress seems premature to me. IRC (#bitcoin-core-pr-reviews) is definitely the best place to ask questions as a new(er) contributor. That channel was set up specifically to help new(er) contributors get to grips with the codebase and the Core review process.