You might want to put pen to paper (or fingers to keyboard) straight away, but you need to do your research first. Start off by rereading the job role and making notes on what the employer is looking for in an ideal candidate. If you tick all of the boxes then you need to outline this in your covering letter, but even if you don’t, it’s worth pointing out your other strengths and noting them down before you start.
It’s also advisable to do your research on the company you’re planning to apply to. Get to know their values and the type of work they like to do. If you have shown a vested interest in them, this will shine through in your covering letter.