Hey!
Congratulations on the interview invite!
For a first round interview, I'd say that the focus is likely to be less on technical legal knowledge and more on your motivation towards the firm/work and your ability to operate in a professional environment. However, I've found in past paralegal interviews that they can sometimes ask you some brief technical questions based on the area of law / practice area you are interviewing for - for example, I was asked about the difference between freehold and leasehold for a paralegal interview in a Real Estate team.
The key thing is to be very clear on your "why" for various questions - for example, why this firm specifically, why a paralegal role (rather than a training contract, at least at this moment in time), and what you are hoping to gain from the experience. I would try to reference things that have drawn you to the firm, such as its practice areas, clients, or anything distinctive about its work, and find a way to link this back to your skills and past experiences.
You will almost certainly get a few competency-style questions, so it is worth preparing a few strong examples that you can adapt to different questions. I always used to create a mini bank of my experiences, and I would make a note of the competencies and skills that I could discuss in each one - I found this was a useful reference point, and helped me ensure that I was talking about a wide range of experiences. In terms of what to prepare for, I'd think about times you've handled pressure, balanced multiple deadlines, worked in a team, or dealt with a challenging situation. It is important to keep your answers structured and focused, but not too rehearsed (which I appreciate is a tricky balance!).
It is also important to show that you understand what the role actually involves day-to-day, and why you are interested in it. It'll differ depending on what practice area you are interviewing for, but paralegal work will often include things such as document review, legal research, bundling, and general support for fee earners. I would therefore try to demonstrate that you are detailed-oriented, reliable, and organised, ideally with evidence from your past experiences.
In terms of commercial awareness, I don't think they'll expect anything too in-depth at this stage, but it is always helpful to have at least one recent legal or commercial story that you can talk about and, if possible, link it back to the firm's work or its clients.
Finally, I wouldn't underestimate how much they are assessing whether you would be pleasant and easy to work with. I conducted a few paralegal interviews during my placement year, and being personable and engaged can make such a big difference and leave a really strong impression!
Wishing you the best of luck - I'm sure you'll do great!