If it’s a vacancy you’re applying for, say which vacancy and where you saw it advertised. If it’s a speculative application, say what sort of position you’re interested in. Briefly and succinctly say why you’re suitable for the job, referencing bits of your CV that you’d like them to pay particular attention to. Make sure that you’ve tailored your CV and covering letter to the company you’re applying for, rather than sending a generic application that reads like you could have sent it to anyone.
Don’t make it too long, or just repeat the bulk of what your CV says, and make sure you proofread it. One of the quickest ways an application gets binned is if it contains multiple spelling mistakes, grammatical errors and typos.