Start by recording your own voice. You can say anything, but make it long enough to have interesting shifts throughout the clip. I recommend using Audacity to do this, because its minimalist interface really streamlines the process of voice recording and editing. Once you have your clip, select sections of it, and add effects. Raising or lowering the pitch, changing the speed, and adding wavers are effective ways to distort your voice in interesting ways. Also, take copies of sections and layer them over the originals, editing those and setting the timing off kilter. That’s an excellent way to easily make your voice very creepy!
And here’s one that I made up