I was able to learn enough of that to be able to read, write and speak to manage 3 independent weeks in China. Effective communication in spoken Mandarin was the hardest, because of the tonality
This recalls a
syndicated agony column from years ago: 'Anyone who can eat around a worm and carry on a conversation with the hostess at the same time doesn't need advice from Ann Landers.'
Anyway, you'll find any Chinese characters you can recall to be highly beneficial. Ok, some have slightly different forms or usages in Japan, but there is a high degree of similarity.
There is lot of good advice above but here are a few pointers that come to mind from a Chinese-and-Japanese speaker:
* Mandarin is hard at the beginning because of tones and unfamiliar sounds. After you have got thru that bit, it is more straightforward. Putting words together into sentences is simpler than European languages but because of the lack of grammatical endings. Conjugate the verb 'to have' in Chinese: '有 / yǒu'. Job's a good'un.
* Japanese pronunciation and word stress are not hard. It must have one of the simplest sound systems of any major language. Learn the five vowel sounds and how to make the 'o' and 'u' long when necessary (and it is a looong sound). Keep the stress patterns even and keep practising the polysyllabic words. There's a bit more to it than that, but not much.
* Japanese has a number of conventional polite expressions that get used a lot - and I mean a lot. When you're in Japan, you'll hear them all the time and so can work out when to use them. 'Sumimasen' is a good all-purpose polite expression. It can mean 'excuse me (can you tell me where the station is)', as well as 'I'm sorry, a mild 'thank you', and a mild 'see you' ('arigatoo' and 'sayonara' carry more meaning and so are not always the most fitting)
* Japanese has lots and lots of words from other languages. Those from outside the Sinosphere are written in the katakana script. Japanese will expect you to understand 'katakana words'
aka Gairaigo, even those that are
meaningless to English-speakers. The good news is that most are not meaningless: for every 'pasu-kon' (personal computer) there are ten or a dozen on the lines of 'miruku', 'rajio', 'pen', 'doa', 'aisu kurīmu' etc. Learning these is a good language-hack for Japanese.
I could go on with more if you like, or PM me if you have specific questions. In case you're wondering, I studied Chinese at Uni (Leeds and Beijing) and lived in Japan in the 1980s.