The Portuguese believes his side have a good chance of beating the Blaugrana and says that home advantage could be the difference between the sides
Los Blancos beat Getafe on Sunday night in La Liga, with the Portuguese scoring a hat-trick - and his 300th club goal - to help his side to a 4-0 victory.
And Ronaldo believes Wednesday's game provides a good chance to record a first-leg lead against the Blaugrana.
"We will be at home, in our stadium which is a good opportunity for us to take an advantage in the tie," he told AS.com.
"It's a really important game, a difficult game. We will be up against one of the toughest teams in the world so it will be a great night.
"We are missing a few key players - players who are important to us but that shouldn't be an excuse. We need to put in a good game because Barca are an amazing team."
Ronaldo also admitted that los Blancos would have to improve against Barca following their first-half display against Getafe.
"We were a little sleepy to start because we are not used to getting up so early to play. We were a little slow in the first half and we didn't play that well," he said.
"The coach's half-time advice helped us and we did a good job in the second half. We scored four goals and that is important for team moral and we're ready for the games we have coming up."
But he played down his own three goal contribution - the 19th hat-trick of his Real Madrid career - which leaves just Telmo Zarra and Alfredo Di Stefano ahead of him, with both players scoring 22 three-goal hauls during their time at the Santiago Bernabeu.
"It's good to reach that mark [scoring 300 club goals], I am happy about it. But my job is to keep doing what I am doing and do my best for this club.
"I want to beat my own record, score more and be better than last season. I constantly try to give the best of myself."