When you start sending out your portfolios, you’ll want to include a cover letter. That said, I don’t think many creative directors make time to read cover letters. So I’m offering the advice I got in portfolio school, which I’ve used ever since:
1. Tell them why you like their agency.
2. Tell them the job you want.
3. Tell them why you think you'd be a good fit.
4. Give them your URL, PDF or however you're showing your work.
5. Tell them you'll follow up with a phone call in the next couple of weeks.
Pretty simple. And I’d shoot for a word count of under 120. (Exactly the number of words that are used in this post.)