I’m assuming you are possibly a high school student? There are a lot of great ways to establish a portfolio. Yes, a blog is one of them, but getting involved with a school newspaper is also an excellent option.
I have friends who were journalism grads in college that succeeded in successful careers as sports journalists, news or editorial writers, and even someone who turned his journalism career into his passion as a theater critic.
I wanted to be a TV journalist and got involved in the campus radio station when I was in college. There are plenty of other opportunities to use use your skills too - the point is to use them! I highly recommend internships, as well. Apply for opportunities at local TV stations, newspapers, anywhere that enables you to get relevant experience and that provides you something for your portfolio. Mine was in a TV studio.
My other piece of advice is don’t ignore the basics — things like spelling and grammar, geography, history, research skills — all of these are extremely valuable in your career. Having a command of the basics is essential — even in my career now. As a hiring manager, I’ve gone through resumes looking for “the basics” like spelling — don’t let spelling or grammar be a detractor.
My other piece of advice is to be a lifelong learner. Technology is constantly changing - as it has in my own career. Don’t ever assume that you know everything - always strive to learn more - always strive to seek actual verifiable facts. Don’t assume anything.
I’ve had a successful career in international and national news media. I’ve written for global B2B magazines and have also written a book. If there is anything more I would want to pass along is to love what you do and always strive to learn more, know more, and deliver your best work.