Hey!
It is completely normal to be nervous before an AC, especially your first one! I'm happy to offer some pointers on how I would approach each task below
Written Task and Presentation
I would try to focus on using a clear structure and showing well-reasoned judgement, rather than trying to give a "perfect" answer.
For the written task, try to:
1. Start with a clear recommendation
2. Support this with 2-3 key reasons
3. Flag any main risks, and explain how you would mitigate these
Overall, aim to keep it concise and commercial-focused - it can be easy to slip into "essay-style" writing, but avoid this where possible!
For the presentation, you won't be expected to repeat what you wrote word for word. I would use the presentation to explain why you took that approach and the trade-offs that you considered. You should aim to show your thought-process, and any relevant factors or considerations that you took into account. This could also involve flagging areas where you might need more information before making a definitive decision, or practical steps that could be taken.
If challenged (which happens most of the time, so don't panic that they're doing it because your answer is "wrong"), make sure to talk through your reasoning calmly - it is absolutely fine not to know everything, but talking the assessor through what you are thinking helps a lot.
Group Exercise
This is usually much more about how you work with others, rather than the overall final answer. The assessors will usually be looking for collaborative behaviour and clear contributions to the group - however, this doesn't mean that you have to talk the most! I always aimed to be neither the quietest or the loudest in the room, and found that a middle ground struck the right balance. You could contribute by helping the group stay structured and on time, and helping track the progress as you go along.
It can be difficult to prepare for a group exercise and written task, but you could practise summarising lots of information quickly (e.g. by reading a commercial news story and creating a brief summary), as well as practise explaining your thinking out loud to others (e.g. your friends or family).
Overall, I'd try to go into the AC with the mindset of being a good future colleague, rather than trying to be the perfect candidate. Best of luck, and I'm sure you'll do great!!