Just start to write, for free or not. It goes back to that old adage, “practice makes perfect.” Do you know other creative people? Write something and have them take a look at it and hear what they have to say. By hearing other people’s review of your work would help you determine what you should continue doing and what you should work on. I agree that writing pro-bono would be a good exercise, another way to build your portfolio and is extremely a good deed. You can also start an exercise wherein you look at existing campaigns and create versions of them on how you think you can make them better. Of course, when you showcase these during an interview, you have to say that you just did them on your own and you’re not in any way involved with the real campaign. Working as an intern is a great way to get ‘real-world’ experience. It would be an opportunity for you to get practice plus be able to observe what other copywriters do.