Portugal are in Group G alongside Germany, United States and Ghana, with Ronaldo's participation in Brazil in doubt due to injuries to his knee and thigh.
The Real Madrid star missed Portugal's 0-0 friendly draw with Greece on Saturday, but is said to be relaxed over his fitness problems.
Portugal have progressed beyond the group stages of a World Cup on just three occasions and Ronaldo knows Paulo Bento's men will need to be at their best to do so for a fourth time.
In a statement via his official Sportlobster account, Ronaldo said: "We left for the World Cup in Brazil with hope, but also making sure that we keep our feet firmly on the ground.
"We have ambition, but with the awareness that we have to set ourselves some goals, step by step, game by game. Calmly but with confidence.
"We are not favourites, indeed, but we will, as always, try to do the best possible.
"We know that we have a difficult group stage. In fact, in my opinion, it's the strongest group of the World Cup, but let's take one step at a time.
"We know we have an important match against Germany, the first one in the competition, then against Ghana and finally against the United States.
"Our goal is obviously to pass the group stage. Then we'll see."
Portugal open their campaign against Germany on June 16 in Salvador.
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