Writing a cover letter when you’re applying or looking for work is a great idea (unless you’re specifically requested not to).
If your resume or application is the main course, then your cover letter is the appetiser.
It allows potential employers to see a glimpse of you. Shows that you recognise the skills and attributes that they’re looking for in order to do the job, and confirms that you have them.
A great cover letter could be your ticket into a prospective employers interview pile.
Youth Central have put together a “How to write a Cover Letter” document. It’s well worth a read.
There are free cover letter templates online, in Word, and Youth Central also have a selection listed in the resource linked above.