Create a portfolio with concepts that you can link to from your resume.
As an entry-level copywriter, you most probably do not have a lot of professional experience. But this should not stop you from showing clients what you could potentially do for them. Create a marketing concept/campaign with copies for some of your favorite brands (or create a brand), and use that for your portfolio. A portfolio is optional but will differentiate yourself from other entry-level hires who won't have much practical experience. It also shows initiative on your part and also shows clients the strength of your copywriting skills.