Graphic Design Portfolio

Hello everyone,
I have an interview on the 14th of this month for a Graphic and Digital design course. I have to have a portfolio prepared. Can anyone give me any advice as to what my portfolio should include and how much work I should have?
I've already done a few bits and pieces but I was to know if I'm on the right track and if I have missed anything that would be useful.
I've never studied Graphic Design in great depth so I hope they're not expecting my work to look really professional 😞

Thanks in advance!