A few points to consider,
- if it is reply to an advert answer all points written in the ad
- Try to set yourself apart from the rest, generally if you are applying you have the skills they are looking for, but show what you will bring with these skills.
Typical layout would be something like
Intro Para - I wish to apply for this job ________. I am ________
2nd Paragraph - discuss how you meet the key criteria that they require
3rd paragraph - Other points
Final - Thanks I'm Keen, please contact me on _____ and if you have any questions.
Regards, ____