To learn another language, I think that you need to know (a) the grammar and (b) the culture. So I would like to share this cultural note: Here in the United States, men usually do not use the word "fabulous." If a man uses that word, some people will laugh or look at that man in a strange way. If you come to the States to visit or study, you might consider NOT using that word. There are many jokes about men who use that adjective.

They both pretty much mean the same thing ("great"), but are said in different contexts, with different connotations. "Great," "Excellent," "Amazing," "Wonderful," "Fantastic," "Spectacular," and "Astonishing" are pretty generally used synonyms with very little connotation. "Astonishing" is used least commonly, and is thus the strongest. "Great" is most common, so it is the least strong. In some contexts "great" can be interpreted as not being strong enough. It is used sarcastically sometimes. "Stellar," sounds like it is coming from a teacher. "Marvelous" can come off as aristocratic. It is sometimes used with a fake british accent in the phrase "It's simply marvelous," with a hearty guffaw to parody someone who is rich or pretentious. "Magnificent" similar, but the connotation is weaker. "Fabulous" is sometimes used in making fun of gay men. "Fantastic" is used commonly in writing, but in speech, it can have this connotation as well, albeit not nearly as commonly as "Fabulous." "Cool" "awesome" and "sweet" are all used commonly among young people colloquially these days, and is sometimes used to make fun of stupid young people (as in "Duuuude. That's awesome"). It is also used in poetry. In the poetic context it is very strong, and the connotation of its use by young people is ignored, as that usage only arose recently, but its poetic use has been around for a long time. "Cool" is most common, so its colloquial connotation is weakest. "Sweet" is probably least common, so its use is markedly colloquial. It's hard to know all of the connotations that come with words meaning "good" in English, so if you just want to give generic praise to something, use "great," unless you're confident that you understand the context. Also, I'm from the U.S. Speakers from other places have different connotations for these words.

Marvelous: Extremely good or pleasing. Fabulous: Extremely pleasing or successful. Fabulous prizes. A beautiful spot with a really marvelous view of the mountain tops.