The Gunners face a daunting task at the Allianz Arena after conceding three away goals in the last-16 first leg at the Emirates Stadium, with Lukas Podolski's second-half strike giving Arsene Wenger's team something to cling on to in Bavaria.
"We need to score an early goal," Giroud told German sports magazine Kicker.
"In football you never know. We need to believe and stay focused.
"But we need to correct our bad defensive behaviour as well."
Toni Kroos and Thomas Muller put the visitors 2-0 ahead after 21 minutes in north London before Podolski pulled a goal back.
However, Bayern scored again as Mario Mandzukic restored their two-goal lead, the scoreline meaning Arsenal must win by three clear goals and not concede to be included in Friday's draw for the last eight.
Giroud believes Bayern's approach will give Arsenal opportunities in attack.
He said: "I like to play against German teams because they are attacking all the time. There I get myself more possibilities to score.
"It will be an interesting game for sure."
Podolski hopes to play against his former club despite revealing on Twitter he has a slight injury, while Bacary Sagna is a doubt.
The Gunners will need their best players to be on top form to overcome Bayern, the runaway Bundesliga leaders.
It could be their last Champions League fixture for a while unless fifth-placed Arsenal can overcome a five-point deficit in the Barclays Premier League and climb into the top four in the final 10 games of the season.
The initiative was handed to Chelsea and Tottenham by recent defeats in London derbies, and the position - and another season without a trophy - has led to calls for a change of manager, with Wenger's position under scrutiny.
Giroud said: "Of course we still want make it into the Champions League spots. We are confident, even though we recently lost the derby against Tottenham.
"We've had bad results - in the FA Cup (losing to Blackburn), then in the derby.
"I don't know anything about the coach's future, but he has proved that he is really a good manager."