When you work with a language like Japanese where the writing system is different from the Latin alphabet, transcription is mandatory. There are several transcription systems for Japanese and anyone learning the language ought to know about them.
But first of all, when do you need transcription? When you translate or adapt from a Japanese source. There are always several possible translations and thus several possible transcriptions: you have to take into account the audience, the context and the aim of that transcription.
Let's have a quick overview of the main systems.
The Hepburn system is by far the most well-known and used internationally. It's said to be close to the Japanese phonetics but I don't fully agree on that. Anyway, it's the most practical one when dealing with people who don't speak Japanese. The consonants are based on the English language but the vowels are closer to Italian so you may have to explain the pronounciation.