because of (preposition), e.g. Because of the rain, the match was called off.
because (conjunction), e.g. Because it was raining, the match was called off.
If the adverbial clause comes before the verb it modifies, put a comma after it. If it comes after the verb, there is no punctuation mark, e.g.
Because he was sick, Tom didn’t go to university.
Tom didn’t go to university because he was sick.
The “because” starts an adverbial clause of reason and cannot form a sentence on its own. It must be part of another sentence with a principal Clause.