Hey there :-) Don't worry too much about open days. My main tip would be to just put yourself out there and ask as many questions as you can, especially if there are current medical students leading tours/hanging about. The whole point of an open day is to find out what studying medicine at that uni would be like and the best people to give you insight into that are the current medical students!
I realise that being proactive with questions might seem daunting if you are shy but I'd really encourage you to step out of your comfort zone and just go for it! Also don't worry if you are currently quite shy, many people are/were at your age and it is something that you can definitely improve on :-)
Best of luck and feel free to ask any questions regarding medical applications!
Alex, 4th year UCL medic
6med