Go to the neartest Lawson konbini. Usually near the cashier or the magazines there is a computer where you can buy concert tickets from. It's all written in Japanese, but you can always ask someone to operate it for you. There you'll find tour dates, locations, prices... everything you need. And you can buy the tickets too!!
Or you could just go to the band's website, check out their live schedule, and then go to a Lawson to buy the ticket.
Remember: always buy your ticket at least three weeks before the gig. Clubs are small, there are a lot of music fans, and the tickets usually go sold-out in a couple of days. Even gigs in huge livehouses like Shibuya AX hold 1200 persons, not more. Trying to get a ticket at the livehouse the same day of the concert is almost impossible...
Summer Sonic and Fuji rock festival are the biggest festivals near Tokyo, and are held in july.