Hi Cem,
Welcome to the forum!
I work as a proofreader and in my reference book on punctuation (How to Punctuate by George Davidson), which has one chapter for each punctuation mark, the chapter on commas is by far the longest in the book. Although there are places where commas are definitely needed, there is a large amount of variation in their use among writers in English. Even the use of a final comma in a list, known as the series comma ("apples, pears, and bananas" or "apples, pears and bananas"), has been argued over, with Americans preferring the series comma and British preferring no series comma - but not always!
For "because" (and quite a few other words such as "if", "when", "as"), a general rule is that if it is at the start of a sentence, you use a comma ("Because the sky is high, it blows my mind" - The Beatles. "If I were a good man, I'd talk to you more often than I do" - Pink Floyd). If it isn't, then don't use one ("I'll finish this message soon because it's getting late" and "I'll be sad if you don't reply to this message"). It's acceptable to break these rules if you know what you are doing and you are consistent, and occasionally I will add a comma before a "because" if the explanation is really long, just to give the reader a quick pause. However, you won't go wrong if you follow these rules and it will keep your teacher happy.
There are many many other rules/guidelines on commas so I can't give you a full explanation, but next time you have a comma question, I'll do my best to answer.
