Large groups will cause nothing but issues, just saying. However I you are talking in over a years time or more then most will have dropped out (sorry to say it but probably true)
A basic guide to planning a trip, do them in order.
1. Get the money - Roughly you need about £2000 for 2weeks and 3000 for a month at least to be able to buy a few things, based on a very basic hotel, not buying anything really expensive and mainly going to cheap attractions.
2. Do you need a passport? Visa? Insurance? Health? Time off school/work? Will you have to pay rent/bills on your home while you are away? These are important things not to overlook! Make sure this is all sorted. Do you need to tell people where you are and have emergency back up plans for getting back?
3. Flights, where from? to? How much? If it is cheaper at a diff time/day can you move part 2 to fit that? Also a good time to think how will you get to and from the airport on the day as it may be crazy times and you dont want to waste precious holiday money on taxis doing that.
4. Accommodation. Hotel? hostel? Rented Apartment? Alone? Sharing? spend time sorting this out. a bit of Japanese language will go a long way in getting cheaper things. Decide now which cities you want to visit and if you are staying in more than one place.
5. Convert your money. do NOT do this at the airport, shop around for the best deals at money conversion shops in your town. If the exchange rate suddenly peaks in your favour do it then for all the money you have so far, you can always get more later.
6. Plan roughly what you want to do each day and where you want to visit, have print outs and a get good directions to all the important places ready.
Don't forget how are you getting you and your bags to and from airport/hotels. Dragging a heavy case very far by foot is not fun.
7. Current airport and import/export rules, be safe don't bother with any liquid on the plane at all. It is not worth the fuss they make these days. Even if we all know DS isn't going to make the plane crash, declare it as a mini laptop at the scanning machine, let them look at it, and don't turn it on till they say you can.It is best at airports and in foreign countries to just air on the side of caution and obey every single tiny little rule and custom just to make sure! Certain medicines ain't allowed into Japan, worse some are seen as illegal drugs including sudafed type cold medicine.
Think about your travel outfit, be comfy, nothing with metal all over it to set off scanners. something you can easily take the jacket off or similar if they ask.
8. Travel phrases. Being able to say このネコはカワイいいいいいい。（＾＿＾） may make you happy and impress your 0-speaking friends but will be pretty useless when you need to check-in to the hotel, ask someone which train goes to Shinjuku and various other real world scenarios. It can be seen as boring but take the time to learn a few handy things and it will go along way.
Be well prepared and the trip will go well and you will have a great time
Don't spend forever on step 6 and 8 and miss out crucial things from step 2 for example, do it just about in this order and you should be fine.