Read more about the organization and position in question, talk to peers and seniors, Google what others have to say about it.
You need to sound motivated about the role you are applying for. If the employer doesn't believe that you will contribute positively, they are not likely to hire you.
Write to them about your strengths and capabilities, past experience, future goals etc.
Strike a balance between covering all the important points mentioned before, and maintaining a word-limit that is easy to read.
Don't take everything they say at face value. If you have questions or concerns about the organization or role, you should make it a point to highlight these.
Once you are done penning your cover letter, send it to a friend, an industry expert or someone you trust.