To see the "Good Ending" where Yuna and Tidus are happly together again, you don't have to finish the game at 100%, but you have to do some specific things during the game:


01 In Chapter 3 visit Chateau Leblanc at Guadosalam and speak with Ormi and Logos. They will offer to show Yuna a series of movie spheres. Once this event is over, Maechen will appear again. Speak with him for some background info on the storyline.


02 After defeating Ixion at Djose Temple during Chapter 3, Yuna will fall down the hole in the Chamber of the Fayth and end up on the Farplane. After watching a cutscene with Shuyin and obtaining Crimson Spheres from Nooj and Gippal, she will be alone in a dark area. After another cutscene with Yuna yelling out loud, start pressing the X button repeatedly. You’ll hear a whistle. Keep pressing the X button until you hear three more whistles. A familiar-looking silhouette will lead Yuna out of the Farplane.


03 After defeating Vegnagun, a cutscene will occur where Yuna and company will walk across a field in the Farplane on their way back outside.
Press the X button to hear a whistle. Bahamut’s Fayth will appear after you hear this whistle. Answer "Yes!" to his question, and you will see the "Good" ending.


Moreover, if you have finished the game at 100% and you have also did was is written over here, you get the "Perfect Ending", that is pretty much the same as the Good Ending, with an additional scene thrown in at the end, where Yuna and Tidus speak to each other at the Zanarkand Ruins (this is just a cut-scene, not an FMV).


Also remember that all the endings will appear AFTER the credits.