Shadow of the Colossus/Valus - Colossus 1
 Reaching Valus
The first Colossus that you encounter is often called the Minotaur, but his proper name is Valus. To get to the battle, simply head south from the Shrine of Worship until you get there. This part of the game serves as a short tutorial on how to move and control your character. Once you reach the valley, a cut scene will occur as the Colossus walks past you. The battle then begins.
 Defeating Valus
The Colossus has two weak points. The first is on the back of the left leg, which when struck will cause the Colossus to fall to his knees, allowing you time to climb up onto his back. The second is on the very top of his head, and is where you will do the most damage that you can to the beast.
Although armed with a huge hammer-club, Valus will only attempt to clobber you with it if you run too far away from his hooved feet. He will then swing his bludgeon at you, which causes considerable damage.
The most direct means by which to defeat this Colossus is to chase Valus from behind and catch hold of the furry part of the back of the left foot area. Be careful, though, as the slightest step of the foot touching on the ground can cause damage to you, and send you tumbling due to the tremor generated.
Once you're on the Achilles heel area, climb up to the weak spot and stab it with your sword a few times. Valus will then roar in pain and go down to his knees. This is your opportunity to move from his leg to his lower back, but do it quickly, as he only remains down for a few moments.
Across the back of the Colossus are a few platforms, each one capable of being used as a resting point to recover your stamina. Be careful if you decide to rest on them, as the colossus will shake its back in an attempt to knock you off. If this happens, you'll either need to hold R1 and hope you catch the platform's edge or jump up and grab some fur. If you fall you'll have to start all over again.
Continue up Valus' back to the top of its head. Once you are there, whip out your sword, and stab until he dies. Two or three "full-power" thrusts should do it. If you start running low on stamina, just try to either get onto his upper back or shoulders, or even onto the nearest platform, and stand up again to recover. In terms of difficulty, Valus is one of the easiest, once you understand how to get onto his back.
 Hard Mode Strategy
On Hard Mode, Valus doesn't die after being stabbed in the head. Instead his weakspot disappears and you'll need to find the next weakpoint. This won't be too hard though, since you probably noticed the glowing point while scaling the beast. If not, it can be found on his right arm (the one holding the club). Getting on to his arm takes a small leap of faith; literally. Drop down onto the upper platform on the right. Next run and jump of the edge, quickly pressing R1 to catch the fur as you fall. It's not a hard jump, so with any luck you will end up near his brightly shining weakpoint. Charge up the sword and stab away once more. Unfortunately, the hero's stamina isn't that great, so there's a chance you'll fall before finishing off Valus. If that happens, simply repeat the steps you used to get on him and continue until you win.