My daughter is almost 12 years old, her dad and I share 50/50 custody, since our divorce 2002, neither of us pays child support. We do week to week change overs. She has been asking to live w/ me & her step dad full time more frequently as she gets closer to 12. Her step mom continues to be verbally abusive & does not treat her the same as her 3 biological daughters that she has had w/ her dad. So my question is what is my first step in getting this ball rolling? What does this whole process entail? What are my odds in getting full custody of her?

My daughter is almost 12 years old, her dad and I share 50/50 custody, since our divorce 2002, neither of us pays child support. We do week to week change overs. She has been asking to live w/ me & her step dad full time more frequently as she gets closer to 12. Her step mom continues to be verbally abusive & does not treat her the same as her 3 biological daughters that she has had w/ her dad. So my question is what is my first step in getting this ball rolling? What does this whole process entail? What are my odds in getting full custody of her?

There is a sticky at the top of the Family Law forum regarding children choosing their custodial parent, which can be read here:
https://www.askmehelpdesk.com/family-law/child-choosing-parent-live-376233.html
As you will read, your daughter cannot make this decision. The judge may or may not allow her to testify; the judge in my parents' case wouldn't appoint a GAL or allow me to testify... and I was 17 years old at the time.

The first step would be to talk to your ex and try to come to an agreement which you both sign and submit to the court.

If he refuses to modify the current agreement, then you would have to file with the court for a custody change. Please note that there usually has to be a substantial change from the situation at the time the order was entered; you're going to need better grounds that "Her stepmom doesn't like her and favors her own children."

You state that the stepmother is verbally abusive - how so? Have you addressed this with your ex? Where is he while this alleged abuse and favoritism is occurring?