[ChisaChi]

I'd start with katakana, since it's used for a lot of loan words from English and you'll be able to understand a lot of what you read fairly easily. Hiragana is less useful because most of the time it's mixed with kanji, which takes a lot more work to read, let alone comprehend. Some basic kanji like the numbers is pretty useful, I noticed some restaurants showing the prices in this way.

In terms of speaking, I found sumimasen (excuse me/sorry), ____ wa doko desu ka (where is ___?), and arigatou (thankyou) the most useful. If you're really keen you could read up on some useful phrases for tourists, like ordering in restaurants or understanding directions as well ^_^

I used cash for smaller purchases and card for hotels and big purchases. Card facilities were less common than here in Australia where even small stores have them, but it's not too bad.