As a journalist at the age of 18, you may need a portfolio of articles and you may need a journalism degree from an accredited university as that is generally what companies want as they understand there may be legal issues involved in reporting that may make them liable and they want the journalist to know Media Law and other legal matters in reporting in order to avoid the classic problem of not attributing the true source of facts in much the same fashion of doing a research paper and providing footnotes of your sources. It depends on the type of journalism too. It is possible to provide a portfolio of your own articles or news reports and learn on the job as well.

There is no one-size-fits-all answer to this question, as the credibility of a source and the ability to be paid as such can vary depending on the individual case. However, some ways to increase your credibility as a source may include developing a strong portfolio of your work, being active and engaged in the journalism community, and networking with professionals in the field. Additionally, many news organizations have programs or internships specifically for young journalists, which can give you the opportunity to gain experience and build your credibility within the industry.

I agreee with @Stefan Hearst you need to get portfolio together. See if you can work at a school paper. If they don't have one, then start something. Talk to you teachers. This would look great on any college application.