How do you write a decent cover letter?

In addition to what Mohamed listed, you want to make sure your cover letter doesn't regurgitate your resume. This is your opportunity tell an employer why they need you and what you are a good fit - especially if this is your first job. For example, your resume may list President of XYZ Club but in your cover letter you can say that "You are a great fit for this position because being a club president taught you leadership responsibilities, event planning, delegating tasks, etc".
Because you want to keep the letter succinct, you should right in an active voice. Don't clutter your sentences with extra words and speak confidently about yourself. You can also use a cover letter as an chance to show you researched the organization. Only dedicate a sentence or two to this but you could say something like "I know that XYZ organization has a strong fundraising campaign. As club president I organized fundraising events such as.....and we raised $5000 for our organization"

Do:

Have a strong opening statement that makes clear why you want the job and why you’re right for it
Be succinct — a hiring manager should be able to read it at a glance
Share an accomplishment that shows you can address the challenges the employer faces

Don’t:

Try to be funny — too often it falls flat
Send a generic cover letter — customize each one for the specific job
Go overboard with flattery — be professional and mature