Neil Lennon's side go into their third Champions League group game against the Spanish superstars as massive underdogs.
With four points from their opening two games, there is little pressure on Celtic, but Ledley insisted the Hoops must be mentally strong to face a team hailed as the best in the world.
"We all know what Barcelona are capable of and we all know what a good team we are – like everyone else, we watch them on TV every weekend," said the Welshman. "But mentally it is vital we approach the game as we would any other. We know that we are about to be tested.
"We know most of the players and the level they play at for their club and for their country but we aren't going there to stand back and admire them.
"It is a huge game for us. We want to give a good account of ourselves and see where we go."
Former Celt Marc Crosas is still in touch with many of his former Parkhead colleagues, while striker Miku played in La Liga before his arrival at Celtic.
Ledley will tap into their knowledge, however, he doesn't believe anyone will be able to prepare Celtic for the test that lies in wait.
"We can ask questions but the fact is that we do not a lot about these players already," said Ledley.
"A lot of it will come down to how we cope once the game gets underway.
"It is a really high level and, like in international football, we need to have our wits about us from the first kick to the last.
"It is vital we don't lose an early goal because there are not many teams you see coming from a goal down against Barcelona in the Nou Camp."
Kris Commons, Scott Brown, James Forrest and Georgios Samaras missed the weekend win over St Mirren because of injuries, and all four could be key tonight.
However, Ledley believes that in Victor Wanyama Celtic have a player who is capable of mixing it with the very best.
"Victor was outstanding at the weekend," he said. "He has everything – he is a threat going forward, he is capable of putting in the tackle and winning the ball and he gives his all for the team.
"We know that if we are to take anything from tonight's game then we all need to be at the very top of our own game."