Very difficult to say what you will specifically be doing. This will depend on so many factors - it’s likely to be “diluted” or simple trainee tasks though.
I don’t think there are things you can do to prepare for tasks before your scheme but there is plenty you can do during your scheme:
take clear notes on what is expected
Don’t be afraid to ask questions to get clarity of the task (how long it should take, deadlines, layouts, formats, processes etc). A lot to vac scheme candidates go wrong by not asking enough questions or the right ones.
make sure you don’t burn the candle at both ends too much. You will be learning and working - that’s a tough combination that needs a lot of focus.
Eat well and keep hydrated - keeping your energy levels up will be important
if you are ever unsure, ask a trainee for their views/advice