The first commandment tells us that we need to follow the one true God, that we shall not put strange gods before Him. So what are some of these strange gods that people tend to "worship?" Here are a few that our catechism speaks of:
Superstition: This is an irrational belief that an object, action, or circumstance unrelated to a course of events influences its outcome. Even a perverse excess of religion is considered superstition.
Idolatry: Simply put, this is the worship of other "divinities." These divinities can take on many forms. First the obvious, don't literally worship other gods like Zeus, Mother Nature, animals, etc. Furthermore, do not worship Mary, the saints, or statues (which we as Catholics are often accused of doing). Although Mary and the saints are very important us, and serve as powerful advocates, they are not God. A common form of idolatry is to honor and revere in place of God things like power, pleasure, race, ancestors, the state, money, etc.
Other "gods" which the Catechism warns us of are: Divination and Magic (i.e. astrology, palm reading), Irreligion (i.e. tempting God, sacrilege), Atheism (the belief that there is no God), and Agnosticism (i.e. truth is relative, God exists but he is not revealed, simply making no judgement about the existence of God).
As Catholics, we are blessed in knowing that there is only one God, that he has revealed himself very clearly in the person of Jesus Christ, and that he loves us very much. We choose to worship this one God, and this one God alone.