Hi and welcome to the game!
To play the game you need to set up an account on Yahoo! Japan. You'll probably need to use an online kanji keyboard to fill in the captcha but that should be the worst of it and you'll never have to type anything in Japanese again to play. Keep track of your password, they ask for it every two or three weeks.
The game is browser-based in Flash and you won't need to set anything up to play unless you're trying to use a smartphone, and I can't help you there. If you're using a PC, I recommend using Google Chrome since it can automatically translate news posts. The text in-game, however, can't be automatically translated because it's part of the Flash file, so you'll have to work out where things are with the help of a guide and practice. KHInsider has a guide
here that's still pretty good, and you can search online for more information. There are plenty of tutorials and videos out there, and the Japanese Wiki is
here (Chrome helps here too). Frankly I've been thinking about writing my own guide because most of them out there is coming up on a year old now (the closed beta ended last March, the open beta hit in July) but one step at a time.
Is it worth playing? Funny question to ask fourth. It's a gorgeous-looking game, really does feel like Kingdom Hearts in a way that's hard to quantify, but the gameplay isn't typical for KH. Like most Energy-based games, where you have to wait a certain amount of real time before you're able to play, the gameplay is more about time management and preparation. You don't even really control yourself in combat, just your equipment, your attack cards, and whether or not you spend more Energy to continue a losing fight. Still, it's probably worth trying for a few days to see how you like it.
Officially, the story only impacts KH as a whole on "one line" and we've already been told which line it is (it happens at the end of the Darkball missions). Actually the recent Dwarf Woodlands mission steps on BBS' toes so it's increasingly hard to pretend the game fits into continuity. The general consensus is to treat it like a fairy tale about the Keyblade War, with a lot of embellishments, so it does give a general impression of what the Keyblade War might have been like, like the Factions and Foretellers, even if the details are 80% wrong. You can get a translation of the story
here at KHI, though we haven't gotten the new Dwarf Woodlands missions yet because they only went live less than a day ago!
