There's lots of advice about job interview via Google, but given it's tomorrow, you don't have much preparation/research time. Some popular questions/conversation topics are:-
Name your 3 best strengths and 1 worst weakness (relating to work or the workplace). Try to link the weakness to a strength, i.e. if you say a strength is that you're well organised, you could say that being too organised is sometimes your weakness, i.e. spending too long on planning or research etc. By linking a strength and weakness, it's easier for you to say that you can work on the weakness as it's also a strength!
What challenges have you faced and how did you overcome them. Better if work related, but if not, then other situations, such as school, friends/family, hobbies, etc.
Where do you see yourself in 5 years' time? Show some foresight and ambition, so aim for supervisory/management positions, promotions, etc within their firm, linking to any training opportunities they offer, such as in house training or external courses they'll support you with.
Why do you want to work here and/or why do you want this position. Do some research, make sure you know what they do, review the job specification, mention any benefits such as flexitime, training, prospects, social activities organised by the firm, etc. Basically show you understand the job and understand the firm and match yourself to what they're offering.
How well do you deal with difficult customers and/or colleagues? Think of some examples where you've suffered conflict or challenges with other people (workplace best, but others acceptable) and explain what happened, how you "solved" the problem, etc.
Never tell any lies or make things up. If you can't think of a suitable event/occurrence then use the closest thing you can think of. No harm in maybe a little bit of exaggeration, but make it believable, to yourself at least, and stick as close to the truth as possible so you don't get caught out if any further questions later in the interview link back and you've forgotten what you said (lies are easy to forget, the truth is easier to remember).
Almost certainly at the end, they'll ask if you have any questions. Don't say no, and don't ask about salary! Think beforehand of something to ask them, even if it's something like have any of their previous admins/PAs worked their way up within the firm to more senior positions or in other departments, etc - it's a good opportunity to get across your ambitious nature (if that's what you want). They'll want to talk about their firm, so ask a simple question or two and let them talk!
My son has just landed a job with a firm a couple of hundred miles away. When they asked him if he had any questions, he asked whether they provided any support with relocation, such as estate agent recommendations for finding a flat to rent, advice on what areas to live in bearing in mind public transport etc. He did that to get across the image that he was already looking to move for the job, so was serious about it and thinking of the practicalities. He said they seemed delighted he'd asked that and talked quite animatedly about how they help people relocate, in terms of financial and practical support!