How to I write an article suitable for publication in a national newspaper

People who do that are called "journalists" (except by some politicians).

Journalism is a profession that you can major in in schools.
My suggestion is to search online for the term "Journalist education" or something like that.
For example: https://learn.org/articles/What_Education_is_Required_to_Become_a_Journalist.html

https://www.google.com/search?q=How+to+write+an+article+for+publication+in+national+newspaper&oq=How+to+write+an+article+for+publication+in+national+newspaper&aqs=chrome..69i57j69i60.16295j0j7&sourceid=chrome&ie=UTF-8

Read lots and take good notes.

To write an article suitable for publication in a national newspaper, you will need to follow certain guidelines and ensure that your piece is compelling, well-written, and relevant. Here is a step-by-step guide to help you:

1. Research the Publication: Before writing the article, familiarize yourself with the target newspaper. Understand its style, tone, target audience, and the types of articles it typically publishes. This will give you an idea of what kind of article to write and how to frame your content.

2. Choose a Relevant Topic: Select a topic that is current, newsworthy, and of interest to a wide audience. Ensure that it aligns with the newspaper's readership and covers a subject that hasn't been extensively covered already.

3. Write a Strong Headline: Craft a concise and attention-grabbing headline that summarizes the main point or angle of your article. A great headline will attract readers and make them want to read further.

4. Develop a Compelling Introduction: The opening paragraph should hook readers and provide a clear overview of what the article is about. It should entice readers to continue reading by presenting a captivating angle, posing a provocative question, or providing a compelling anecdote.

5. Structure your Article: Organize your article into coherent sections, including an introduction, body paragraphs, and a conclusion. Each section should flow logically and transition smoothly to keep readers engaged. Use subheadings to break up the text and make it easier to read.

6. Provide Solid Evidence: Back up your claims and arguments with credible sources, such as expert opinions, research studies, or official statistics. This adds credibility to your article and makes it more convincing.

7. Write with Clarity and Precision: Use clear, concise, and jargon-free language to ensure that your article is accessible and understandable to a wide readership. Avoid overusing complex words or technical terms that might alienate readers.

8. Inject Personality and Voice: While maintaining objectivity, let your unique voice shine through in your writing. Inject personality, be engaging, and write in a way that connects with your audience. Remember to remain professional and respectful.

9. Edit and Proofread: Review your article multiple times for clarity, spelling, grammar, and punctuation errors. This step is crucial to ensure that your article is polished and error-free.

10. Submitting your Article: Check the submission guidelines of the specific newspaper you are targeting. Some newspapers have submission portals on their websites, while others may require you to email the article to a specific editor or department. Follow their guidelines carefully and include a brief cover letter/email explaining why your article would be a valuable addition to their publication.

Remember, the competition for publication in national newspapers is fierce, so don't be discouraged by rejection. Keep honing your writing skills, revising your articles, and seeking feedback to improve your chances of success.