In general, if you are talking about a large, general group, you should use most.

Most people know how to ride a bike.

Most doctors earn a lot of money.

If you are talking about a smaller, more specific group, you should use most of.

Most of the people in this neighborhood are married.

Most of the doctors at this hospital earn a lot of money.

Grammatically, most is not followed by a determiner (a, the, this, etc.); you should use most of. If no determiner is used, just use most.

Most of people like chocolate. (no determiner, so don’t use of)

I like most of fruits. (no determiner, so don’t use of)

Most of my friends like movies. (my is a determiner, so you should use of)

I want to see most of the movies playing at the theater right now. (the is a determiner, so you should use of)

Of course, there are always exceptions:

If you are talking about a specific country or region , use most of.

Most of Africa is very hot.

Most of Europe uses the euro.

Note: for more detail, refer to Practical English Usage by Michael Swan