How do I write a cover letter?
The basic structure of a cover letter, is 3 paragraphs consisting of;
The Intro - where you explain how you found the company and the position.
A 'why me' - where you explain how you match the job description and why you would be the perfect match, and finally.
A 'why them' - which is why you have chosen the company, explain the research that you have done on them, and how you fit the company.
But if you are stuck on how to start your cover letter, or write your 5th one, you can have a look through these articles;