'Must' is a word that I often hear over used.
My advice is to try and use it as little as possible. Instead, use 'have to'.
Why is that?
'Must' is a very strong word and so if we use it too much it can lose it's force and meaning.
Native speakers use 'must' only when something is very important or critical and it is used to show the importance of the activity.
'Must' is personal and we use it to show our feelings. We can use it in sentences such as:
"I haven't phoned David for ages. I must phone him tonight." (this is critical to me)
"I must get up early tomorrow. There are a lot of things I want to do." (again, this is critical to me)
'Have to' is impersonal and is used to talk about facts rather than personal feelings.
We can use it in sentences such as:
"My eyesight isn't very good. I have to wear glasses for reading." (fact, I wear glasses)
"George can't come out with us this evening. He has to go to work." (fact, George is going to work)
Sometimes it's possible to use 'must' or 'have to' in a sentence. For example:
"It's getting late, I must go now."
"It's getting late. I have to go now."
In this case using 'must' is a much stronger expression and I would expect the speaker to leave immediately.
Using 'have to' is not as strong and the speaker may stay and talk for another ten minutes before leaving.
If you are unsure whether to use 'must' or 'have to' it is generally safer to use 'have to'.