One of the keys to success in the business world is understanding what you are supposed to do and then doing it well. By the time the offer has been made and you've accepted you should have a good idea of what the job is about - because you asked about it during the interviews. But you may not have the whole picture. That's ok. Don't worry if it takes a couple of days or even weeks to figure out your responsibilities.
Start with your job description (if you have one) and refer back to it often. Ask your boss or your peers if you are uncertain or confused. Ask the people in other departments that you are working with how you interface with them, what they expect from you, and what they are going to provide to you,
You'll need to ask people and maybe you'll find that awkward. Don't ask "So, what am I supposed to do with this form?"
Ask: "It's my understanding that I'm supposed to fill out and file this form with Accounting. Is that correct?" Or, "Should I fill this out and file it?" Or, if you have no clue "What is this form for and what should I do with it?"
It's also a good idea to document your understanding of the job and the things you are doing, even if the company provided you with a written job description. Responsibilities can grow (and shrink) and when it comes time for your evaluation, you'll want a record of what you actually did, rather than a generic job description.
Last point: One thing everybody hates to hear from someone else is "That's not my job." If it's something trivial and you don't care, you can say "I think that's Fred's responsibility, but I'll be happy to do it."
If you are asked to do something you don't think is in your domain, say "I'm not sure that's my responsibility. Is there someone else who is supposed to do that?"
If you are asked to do something you know isn't your job, try "I'm pretty sure that's not for me to do. Can I check with my boss and get back to you?"
