Shelves in cupboard?

Hi there...am new to DIY and would like some help and advice on the following...

I have a built in cupboard in my bedroom that doesnt have shelves in it. I want to put couple of shelves in it..The walls are plasterboard walls. I think I need support on three walls so that I can put the shelf on those. But the difficult part for me is to decide which screws for fixing the support!...

any help will be appreciated...thanks