When you reach the Thunder Plains you’ll be given a quick tutortial of what to do. Press X just before the flash of light and you’ll dodge the lightning bolts, and make sure to dodge them carefully…you’ll receive a prize for your efforts. Also watch out for Larvas, they are mightily annoying and hard to defeat with one character. Summoning Ixion is best in this area since it can never die due to the frequency of lightning attacks the enemies cast.
Note that you can get Kimahri’s best weapon here. You must check all three of the Qactuar Stones and then follow the Qactuar spirit to a treasure chest. Midway through your journey Rikku will become horrified of the lightning storm as she wishes to rest. When you reach the traveller’s inn talk with Yuna. Talk with Rikku and later Rin will ask you how your Al Bhed is coming, select the first option to receive the Al Bhed Primer Vol.XIV. Then head off to the save point and save your game, but as you continue on, Yuna will have finally made her decision.
Continue on and after a short conversation between Tidus and Auron, save your game. Make your way around.