Congratulations, you got the offer! Whether you're accepting or declining it, be polite and timely; the people with whom you interviewed are going to be your professional colleagues.
You must speak with the person who offered you the job. A phone call is acceptable, but you must speak directly with the person who offered you the job. It isn't appropriate to use voice mail, e-mail, or a written letter. If you're having difficulty contacting the person directly, zero out of voice mail and ask when would be a good time for you to call back.
If you're accepting an offer, you may wish to send a brief written letter that confirms your acceptance and reiterates your enthusiasm for the job and for the employer.
Don't forget to contact all the people who helped you in your search to say thanks and to let them know about your new job.