Keep cover letters brief and to the point, whilst including all relevant information......dont start waffling or your letter will get put in the def NO or maybe pile when employers are sifting through the applications. As you can imagine the employers have a lot of applications to look through, so those that keep their attention are more likely to get chosen for the yes pile.
Include any qualifications and/or experience you have for the position applied for. But also include a little about why you are a good person for any employer to have, stuff like....
Well mannered, willing and eager