Problem with Inverse Kinematics - Pole Target?

Hi, I tried to solve this myself but I thought that after all this work reddit was the best option. See, i've made an armature for my character, and i wanted it to have IK arms, but when i try to parent de mesh with the armature, the arm rotates itself automatically, deforming the arm and the mesh... and it's always when i put the Pole Target. Whithout it, it works fine. Does someone know why this happens and how to solve it? Thanks.

Without Parenting

Parenting