Jose Mourinho's side have been drawn in Group D alongside Manchester City, Borussia Dortmund and Ajax for this year's competition, and Ronaldo believes that their group is the most difficult draw in the tournament.
The Portuguese forward said: "It's pretty difficult. And I am sure the opinion of most people is that it is the most competitive group of the lot.
"We are ready. We are champions of Spain and we are ready to compete with anybody."
Madrid collected their first piece of silverware on Wednesday in the Spanish Super Cup against Barcelona.