Every time I play I automatically go into defensive mode. It's like I can only do defense. I never win a game, and I think this is why (or, I am worse at chess than I think).

How does one become offensive?! 

take a more strategic and proactive approach by  trying to control the middle ground of the board, providing more flexible options to launch offensives 

Should I try to use certain pieces? At the moment I am trying to make the most of my pawns... 

pawns are the foot soldiers and need backup by the heavy artilierysuch as the knights and bishops- use these to defend the pawns asthey move forward but also to gain greater strategiccontrol of the middle board and then to launch attacks