You don't need to speak Japanese to enjoy sports in Japan, but knowing a few key words makes the experience much richer. Here are the most useful terms organized by situation.
がんばれ!(ganbare!) = go for it! ナイス!(naisu!) = nice! すごい!(sugoi!) = amazing! お疲れ様!(otsukaresama!) = good job! (said at the end)
Useful signs
禁煙 (kinen) = no smoking. 撮影禁止 (satsuei kinshi) = no photography. 持ち込み禁止 (mochikomi kinshi) = no bringing in (food/drinks).
Summary
Memorize the ticket and stadium words before your first game, and pick up the cheering phrases as you go. Japanese fans will love hearing you use these words.