In the standard cover letter format, the introduction should clearly state the position for which you are applying.
This should not only be the focus of the opening paragraph, but it should also be the focus of the opening sentence. One sentence should suffice for the opening paragraph, but you may want to use an additional sentence to denote the name of a personal contact within the organization who recommended the job to you or to briefly mention your qualifications for the job.
As always, focus on attention to detail. By ensuring the corectness of your resume and cover letter, you will advance past a great number of candidates who were not so diligent.