Another question about evolution...?

just one thing i dont get, and that is how the DNA/genes change in evolution. For example, giraffe's apparently being litle ponies way back in the day... how does them needing to get food up in trees make them evolve? just because one giraffe says i want that food doesnt make the DNA go change itself. This giraffe may have its heart and soul set on getting that leaf, but honestly i dont think it changes ****. And mabye it littles baby giraffe's want that leaf too, and his ancestors millions of years later want the leaf, but it still doesnt change a thing. So what i wanna know, what "chemical" thing changes the DNA to make things suposedly evolve.