A little Japanese goes a long way
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.
Ticket buying
チケット (chiketto) = ticket. 自由席 (jiyuuseki) = unreserved seat. 指定席 (shiteiseki) = reserved seat. 内野 (naiya) = infield. 外野 (gaiya) = outfield. 大人 (otona) = adult. 子供 (kodomo) = child.
At the stadium
入口 (iriguchi) = entrance. 出口 (deguchi) = exit. トイレ (toire) = toilet. 売店 (baiten) = food stall. ビール (biiru) = beer. 応援 (ouen) = cheering.
During the game
ゴール (gooru) = goal. ホームラン (hoomuran) = home run. ストライク (sutoraiku) = strike. アウト (auto) = out. ファウル (fauru) = foul. 勝ち (kachi) = win. 負け (make) = loss.
Cheering phrases
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.