This is not something that easy to wholly understand or explain...but generally you're gonna use wa to mark whatever the action doer is, ga is going to mark anything that is going to be followed by an adjective and some verbs like wakarimasu. anata wa ringgo wo tabemasu. you're gonna eat an apple ringgo ga akai desu. The apple is red. nihongo ga wakarimasen. I don't understand Japanese A little more in depth... wa can be translated as... "for one", or "at least" ga is gonna mark something specific use ga when responding to a question kare wa paati ni ikimasu hes gonna come to the party. (hes not the only one that's gonna do it) kyou dare ga seppuku shimasu ka. ano samurai ga seppuku shimasu yo. who's committing honor suicide today? That samurai over there will commit honor suicide. nani ga wakarimasita ka? zenzen ga wakarimasen desita. What did you understand? I understood absolutely nothing. Remember the answer to a question is almost always marked with ga. Theres exceptions like: doko ni iku? amerika ni iku. but that should be pretty easy to recognize.

Both of your sentences are grammatically correct. As you pointed out there is a topic vs. subject distinction. 'subject' is merely a grammatical function in a sentence. It's the doer of the verb (generally). 'topic' is not a grammatical function; it's a semantic one: it's whatever in the sentence requires the most attention or focus or emphasis. 1.) Ramen ga suki desu. This is a neutral unmarked statement. "I like ramen" 2.) Ramen wa suki desu. This put emphatic attention on ramen, as if you are contrasting it with other stuff. "I like RAMEN (as opposed to the other types of noodles you mentioned)" Because WA has a semantic function, its use largely depends on the context. So it is hard to give a hard and fast grammatical rule on its use. But note that WA can also replace other particles, too. kanji wo naraitai "I want to learn kanji" kanji WA naraitai "It's kanji I want to learn, (none of that other crap you're trying to teach me!)" Of course, it's not always about contrast. The very common "watashi wa sumo ga suki desu" simply topicalises watashi to make clear that of the two noun phrases, the one that functions as the main focal point, or topic, of the sentence is watashi. Likewise in "watashi wa daigakusei desu" or "watashi wa 50sai desu" wa is used to promote the subject of the sentence (typically market with ga to the semantic role of topic (it's still the subject though!). Objects - as opposed to subjects - can also be topicalised. sumo wa shinai kedo, judo wa suru. I don't play sumo wrestling, but I play judo. Here the objects, normally marked with 'wo' not 'ga' are topicalised with 'wa'. The best way to learn it is to listen and read as much Japanese as you can!