Sometimes it’s easy—you just say, “Thanks, I’ll take it!” However, there are situations where you may not want to say that immediately. For example, perhaps you were fortunate enough to line up several interviews, and you get offered job A before you have gone to interviews B and C. You can explain to the employer that you have additional interviews lined up, and that you would like to investigate all your options before making a decision. However, be aware that the employer may not be willing to wait more than a week or two for an answer, so if the other interviews aren’t scheduled close to the date of the offer, you may need to accept the offer or lose the opportunity.
Another scenario where you may want to ask for some time is if you’ve had several interviews, but haven’t received decisions from everyone yet. This becomes important if you think you would really prefer job B to the job just offered. It is perfectly acceptable to respond to an offer by saying, “I’d like a few days to think about this position before making a decision.” You need to tell the employer when you will get back to them (“Would it be all right for me to call you on Friday with my answer?”), and then immediately contact the other employers to tell them you have an offer in hand and ask when they would be able to make a decision.
A third possibility is that you have no interest in the job you were offered. I realize that it’s very hard to turn down an offer if you don’t have another immediate prospect, but you need to think carefully about any position you accept. If you think you can last in the job for at least a year, and you have no other options, go ahead and accept it. But if you think you will dislike the job so much that you plan to continue looking for another position after accepting this one, you are being unfair to the employer and likely earning a negative reference for the future. In that situation, I suggest that you thank the employer for the opportunity, but explain that you don’t think this is the right position for you.