There are two sorts of bow: from the head and a profound bow from the waist.
A profound bow may be used in place of genuflecting, if required.
A head bow is made to the Processional Cross at the start and end of Mass.
A head bow is made the Celebrant in procession at the start and end of Mass (you may genuflect if the celebrant is a bishop.)
A head bow is made whenever the name 'Jesus' is mentioned in church.
A slight head bow may be made for the name 'Mary', the mohter of the Lord, when at Mass.
An even smaller head bow may be made at the mention of the saint of the day, when at Mass.
These are not hard and fast rules: there will be no lightning bolt from heaven if you don't follow these guidelines. They are venerable traditions which unite us with those who have worshipped in the past, and those now in the presence of God.
Don't ever freel that omitting any one of these practices makes you a bad person! Do what enables you to feel close to God and reminds you of his loving presence.