How do I decline a job if I've already said yes?

I have two job interviews this week. I have just gotten back from the first one and was asked to come back for a followup interview before the final decision is made. I feel really good about this job and want it really bad. However, I have another interview tomorrow at a different place. I do not want this job as bad as the first one. If i go to the interview tomorrow and they offer me the job but job number one also offers me the job two days later, how do I tell the second job that I would rather take the first job?