OK, I've been to several so I'll try and give I bit of a rundown.
You don't need any Japanese, although if you can understand numbers that would probably be the most helpful. Very few people actually speak to each other anyway, as it's ludicrously noisy. There is an English support booth I believe, but I've never bothered with it. It sounds like you'll probably be fine though.
With the size, you'll be OK if you plan for it. It's just like a really big market. Download or buy the CD guide before hand, and make sure you know where the people you want to visit will be. If you're just looking, be aware that to some extent they do group vendors with similar categories of merchandise together, so if you find something you like look more around that particular area. At the very least get a map of Tokyo Big Site and get a general idea of the area.
Take lots of water. Tokyo is really hot, and being in a room with 100,000 other people doesn't help.
No preregistration is needed to get in. It's free, and open entry to all. Be aware that the lines when the doors open are super long. Like people line up overnight to make sure they're first to their chosen vendors. If you don't care about that and just want to have a good time, I'd recommend you turn up at midday-ish. The crowds have died down by then and it's a bit more relaxed. People can be real pushy for the first couple of hours.
I dunno, it seems like a big deal, and if you're going over there specifically for it you probably want to plan ahead to make sure you have a good time. But it's really not as confronting as you might think.