The 24-year-old signed a five-year deal at the weekend after moving for a fee of 19 million euros.
"I am coming to the best team in the world and I am delighted to be a Barcelona player," Song told a news conference. "I know I have joined a team of stars and I will have to fight for my place.
"It is a great challenge and I know I have a lot to learn. When the coach [Tito Vilanova] thinks I am ready that's when I'll play.
"It is a once in a lifetime opportunity. I thank Arsenal and [coach] Arsene Wenger for allowing me to be here."
Barcelona sports director Andoni Zubizarreta said the club were attracted to Song by his versatility.
"We decided to sign a midfielder that could play as a centre-back after the exit of Seydou Keita," he said. "Song was the first name on the list.
"He is a player who is good in the air, is physically powerful and tactically astute."
Song said he was willing to play anywhere for a Barca team that has won 14 titles including three La Liga and two Champions League trophies in the last four seasons.
"I haven't come to take the position of [holding midfielder] Sergio Busquets," he explained. "I think he is the best in the world in that position.
"I have simply come to help and play my part. I will adapt to whatever role the boss wants."
Song will be reunited with former Arsenal captain Cesc Fabregas who tweeted on Monday: "Welcome to Barcelona Songito. Wish you all the best mate."
"Cesc is a good friend and has explained many things about Barca," Song added. "But he didn't need to, I knew already because I have seen them on television.
"The style of play is similar to Arsenal's. When you go to a new place you have to adapt and Cesc will help me."
Barca's next game is at home to La Liga champions Real Madrid in the Spanish Super Cup first leg on Thursday.
