Say thank you. You can be polite without giving an immediate response. Let them know that you’re very appreciative of the offer, but would like a little time to make an informed decision. This may be your first interview and offer so its understandable you need time to consider.
Asking for time allows you to ask more questions
Do give a specific amount of time. Up to 7 days is generally seen as reasonable but whatever time period you ask for do stick to your word.
Don’t be afraid. Its not unreasonable to want time to consider an offer especially if you have other offers or opportunities to factor in. If the business rescinded an offer because you asked for a little time then its probably not someone or company I would recommend you work for. Remember that you are committing to a new employer so now is the time to get it right.
Asking for more time allows you to think carefully about your decision.
Above all else don’t burn bridges. It’s a small business world now and your hirer may well come back with another opportunity or can either reappear at another company in the future, so always be professional.
Formally accept or turn down the job. Always accept or turn down an offer verbally and follow up with a polite and gracious email!