1. Put together your resume if you haven't already. If you have one, then you should include it here or at least reference that you'll provide it to interested parties.
2. Have some code to showcase that can demonstrate your ability to take a spec to code completion.
3. Look at other Ask HN posts that are the same topic; see what those users have posted and what the community has responded with. This may come off harsh, but I've seen several in the past week alone, and yours is the least compelling post of them all.
It's not about who you are and what skills you have but more about what you can do for the company.
The reason they want a portfolio is because they'll be investing time and resources into you and while you may be confident in your ability to deliver, they're probably not.
If you end up failing to deliver, it would be wasted time and resources on their part. The point of a portfolio is to show that you have the skills to do what they're looking for.
Don't let the other guys drag you down - they sound bitter. But from their perspective, you're not giving them anything to work with. Fill the gap pronto! :)
Why a junior intern? Why aren't you ready for a full time paying role?