Yes, you absolutely can start down the 12 step path without believing in any God or Higher power. I did it and many people I know from A.A. and N.A. have also done it.
You may find that after several months or several years, your whole outlook on the spiritual life has changed. At least, that is what happened to me. I still don't believe in the God I didn't believe in when I started but I do believe in a higher power I had no knowledge of or experience with until I started.
Honesty, open mindedness, and willingness for sure. And don't close any doors before you get to them.