I think you are getting some useful advice on your other thread, but for a covering letter you need to attract the attention of whoever is reading it, normally within the first thirty seconds of starting to read!
You do not want a long letter just an introduction to your capabilities, qualifications and enthusiasm.
This needs to be enough to encourage them to read the CV.
You are at a potential disadvantage without work experience so you you need to balance this with (truthful) capabilities or experience that you have already had that will appeal to the employer.
Relevant skills gained through hobbies and holiday jobs.
However do not cover any irrelevant skills, this is just wasting your and their time.
Specify exactly when you are available and what you realistically believe you can do for the company.
Put all statements in a positive light.
e.g. do not put "I have no work experience yet", put something like "I am sure my skills at DIY, having rebuilt the family kitchen, will demonstrate my capability to provide you useful asset as a keen apprentice in his first job".
I tend to be over wordy, keep it brief and to the point. Give facts and figures where relevant.
Look on the web for covering letter, these will give you a good idea of what is required in general.
