Just write saying you wish to apply for x vacancy as advertised in/at xxx. Then say something like 'my experience includes blah blah blah....' and then bullet point your best bits e.g. excellent communication skills, good organising skills etc etc and finish by saying you are available for interview on M - F between X & X da der da der da....
You can tell them why you want the job if you like or if you havent already done so in your CV, and mention any activities, eg voluntary work or awards which may be releveant to the job applied for. Overall, try to use words they have used in the ad - it makes it easier for the interviewers to tick boxes!! good luck