'Have to' and 'must' are both used to express obligation. There is a slight difference in the way that they are both used.
'Have to' shows that the obligation comes from someone else, not the speaker. This is usually referring to a rule or law.
We have to be at the airport at least two hours before the flight.
I have to work on Saturday.
They have to wear their uniforms at school.
'Must' shows us that the obligation comes from the speaker.
I must hand in my thesis by tomorrow.
I really must call my parents.