Didier Drogba scored a first-half winner while the Blues spent much of the game defending as the Catalan club enjoyed 72% possession of the ball. However, Cahill insists that they haven't achieved anything just yet.
"It was a fantastic result but it's only half time," Cahill said.
"I thought as a team tonight we were well organised. Everyone was on the same page.
"The gaffer gave us a game plan before the game and we stuck to it. It worked really well, but it's half time and it's going to be really hard over there but it's a good start for us."
Chelsea travel to the Nou Camp on Tuesday for the second leg of the Champions League semi-final.