Best time to go is Spring (March-May), or fall (Sep.-Nov). The temperatures are nicer, there's less crowding at tourist locations, and there are also a number of fun holidays in Spring (Golden Week). The summers in Japan are dreadfully hot and sticky, not to mention its the rain/typhoon season. The winter can get a little too nippy to enjoy many of Japan's outdoor attractions, and travel is very difficult around the New Years.

Well i like cold weather better than hot but i would love to go to the citys and shop and go to the hide musuem.

Then Tokyo in the winter is probably for you. February is probably cheaper than January, when a lot of people travel.

Ok thanks but about how cold does it get?

Like i dont want it soo cold that you cant do anything and i dont want it sooo hot that you have a heat stroke. but is it in the 50's?

Sorry, but most places in Japan, you're going to have both of those. You just bear with it, and spring and fall are nice...

But i love the cold weather. But if i did go in the summer ppl say its really hot and sticky. is that true?

Do you know if it's really humid/hot in the summer in places like Hokkaido?

Never went to Hokkaido, but it's supposed to be cooler there than the rest of Japan in summer (still humid though, I think).

Yes, most of Japan is very hot and humid in summer. Cooler the farther north you go, of course.