If not already affiliated to Hombu, it will cost approximately 8400 yen (42 GBP or 84 US dollars) for the entry fee. There is usually a small box of application forms (in English) on the front desk counter.
As for ettiquette, whilst Sensei is demonstrating a technique or personally correcting you, you MUST sit in seiza - even those poor people kneeling on the hard wooden floor at the back of the mat! Someone will remind you if you forget!
Spring is the best time to visit Japan IMHO unless the humidity arrives early!