We first have to understand a Catholic's relationship to the Bible. Many protestants define themselves through a literal interpretation of the bible. They are people of the book. Catholics see the bible as part of a living relationship where scripture and tradition are divinely inspired. The Bible is our book.
As Catholic we believe the bible is true!! We believe that God does create, sustain and order the world out of nothing through love and mercy (CC 295-327). They proclaim the deep truths of the purpose of creation "God wills... it is good". Adam and Eve make it clear that we created to live in the Garden yet the reality of sin separates us from God and one another. These are deep truths but they are not scientific truths.
Creationism is a pseudoscientific theory of how the world was created came into being in the 1920's was meant to challenge the theory of evolution. For Catholics there is no contest. The science of cosmology and evolution study how we came to be. While our faith answers who and why we are. Scripture is remarkably silent on how God created us. God said and we were created.
Many Catholics have been instrumental in the field of science (remember who founded all those universities). My favorite is, Monsignor Georges Lemaître, a priest from the Catholic University of Louvain, proposed what became known as the Big Bang theory of the origin of the universe, he called it his "hypothesis of the primeval atom".
So Catholics can hold evolution as a scientific theory but it is important to remember that faith is the basis of a moral life not science. To that end, Pope Pius XII addressed the question of human origin more specifically in his encyclical Humani Generis. He stated that the origin our "body" from pre-existing and living matter is legitimate matter for inquiry. Catholics are free to form their own opinions. But Catholics must believe that the human "soul" is created directly by God. Being spiritual substance we are each created uniquely by God.
Our faith allows us to answer the moral question of should I do something rather than can I do something. Some people have made science their religion. This is a mistake because science, while useful, is incapable of answering the deepest, most human questions of life. Redemption is not found in science... no matter how great my i-pod is!! There is no app for that but Christ.