In terms of extra curricular stuff look at sports, musical instruments clubs and societies at school, but to be honest it isn't how much you do, it's what you can say about it, as long as you explain how the things you do help you to be an ideal candidate you should be fine
The closest thing to NCS i can think of is DofE that helps when you're trying to show leadership/communication/organizational skills
As far as shyness goes I don't really know any techniques as such but you just have to think of the benefits of doing it outweighing your nervousness, if you just have a 30 second conversation on the phone then it could lead to doing the career you want for the next 30 years or so, possibly an overstatement but volunteering is reeaally important so I recommend just getting stuck because shyness is something you have to get over if you want to be a doctor or even get just though interviews so it is worth it!
hope this helps, good luck!