Who knows how to power up Tidus's celestial weapon.

You need the Sun Crest and the Sun Sigil (i think it's Sun). Anyways, the Crest is found in the room where you fought Yunaleska in Zanarkand. Look around for a treasure chest and if you can't find it, go up or down the "stairs" and then research the area. For the Sigil, you'll have to race the Chocobo Racer again. However, you'll need to get a large amount of balloons and hit by as few birds as possible to bring your time lower than 0:00.00. Then she'll give it to you as a prize.

Can u tell me how to get 0:00:00 on the chocobo race becuse I would usually get a 0:02:56

Practise, I haven't beaten 0:0:0 yet and I tried alot of times. It just takes skill and luck I guess. There is a cheat for it in the Japanese version but I presume we are talking about the Int. version.

You have to do a few things to get lower than 0:00:00.

First, complete the race n the shortest time possible, so you have a better chance.
Second, avoid all birds if you can. Getting hit once is OK though.
Third, try to pick up as many balloons as possible. Of course, they're randomly distrubuted, so if the first 5 are right next to the trainer you're probably screwed ^-^.

I still haven't beaten it though...

You can get a negative time? I always get 0:00:00 nowadays...never been able to "beat" that lol.

Well, it just shows up as 0:00:00 on the timer, but it'll process it as a negative time.

Make sure to get as many of those early balloons as possible! If you don't get the early balloons, you're probably screwed. Get familiar with the positions of the birdies too. Don't get so focused on the balloons that you get hit by a bunch of birdies.

<font color=red face="comic sans ms">When a flock of two birds fly towards you, two aim at you and two aim at the trainer. In fact, the ones for the trainer can actually go straight through you without hurting you at all. You need to start dodging the ones aimed at you pretty early on; so if you stay far enough from the trainer yet still in line with the balloons you'll know exactly which ones to dodge.

Hitting one bird to get one balloon isn't worth it, but if you'll get 3 balloons then it is. And be careful particularly near the end of the course because there is an invisible border to the left, and if you try to cross it you'll lose control of your chocobo for a while as it tries to get itself back on the course; I hit so many birds through doing this so be careful.