The last thing he remembers clearly is his wife's death, and he vows to hunt down the killer. Because he has no short-term memory, he keeps track by tattooing clues on his body, taking photos of the people he meets, and writing down what he knows about them on the back. See, he won't even remember them the next time he sees them. All Pearce knows is not to trust his lies. Carrie Ann-Moss is also either helping or hurting him, but he doesn't know, since he has n idea if what they are telling him now matches up with what they told him before.
Memento is a noir told backward.
The first scene is the end of the story, and you know who Pearce kills. The rest of the story rewinds to the beginning, revealing the details of how and why Pearce killed the person he did. You see the end of story strands, red herrings and fights, before the beginning. It's supposed to give the viewer the same disorientation as Pearce has with his memory loss. And fuck if it isn't disorienting.