It is advisable to accept, in writing, any job offer you receive. That is, assuming you really want the job. Do not accept an offer you are not sure of. If you are interviewing with several companies over the course of a week or two, be honest with the potential employer and ask for time to make a decision. You do not want to jump on the first offer you get because the next one may be the to-die-for job. Give yourself a reasonable amount of time to make the best decision. Please note, I said “reasonable”. An employer should be willing to give you a few days or up to a week to decide.
In your letter of acceptance be sure to mention the exact position, the salary and the start date, if known.