People are always telling me to 'get involved'?

i am always on the sidelines to be fair but i feel properly awkward and shity around ova people - i wasnt like this before but i really cant snap out of it Im 19 and have no mates - people who do invite me out with them get fed up of me when they see my shy boring side, its frustrating cos i will talk and be myself with em sometimes how can you sorta stabilize your personality