When I write cover letters, they're ALWAYS tailored to the specific firm or job I'm applying to. Usually, my resume is as well because I like to keep it relevant.
You can probably find a number of good templates online, either through places like monster.com or university coop/job placement web sites. Regardless, it should quickly identify what job you are applying for, your qualifications, and the timetable on which you can start; if you're in school, for example, you may indicate that after XXX date, you will be available to start.
Likewise, if you've had any contact (hopefully good) with the person to whom you are writing, you might reference that "we spoke at XXX on XX date."
The whole thing should be maybe two full paragraphs and an extra line or so. But by far, the MOST important thing I can say for resumes and cover letters is that they be ABSOLUTELY PERFECT!!!!!!!!!!!!!!!!!!!! NO grammar or spelling mistakes, because if the HR weenie (and most of them are weenies) happens to spell or write better than you, they can often use that as a reason to throw you out.
Actually, as a reference, this answer to your post is too long for most cover letters
Since you didn't mention what kinds of jobs you're applying for or what your qualifications are, I can't give any more specific feedback--mebbe you've got family or friends who can look over your letter and resume?
Good luck!