Father wants to keep his kids

My wife and I were married 4 and a half years ago and have been together for over 7. We married young and started a family quickly. I have three children ages 3, 4, and 6.
One day my wife decided that she wasn't happy as a wife anymore and she decided that she wants to leave me. She is out partying and acting like a teenager again. We were living in South Florida after moving there two years ago for employment. I am an aircraft avionics technician and she was a homemaker. Now I have been forced to quit my job in Florida and move back to our home state of Michigan just to be with my kids. I want nothing more in this world than to have my kids with me. I still love my wife but she is moving on and we are heading toward divorce. I am now looking for work in Michigan and she is working as a waitress at Dennys.
My question is, what do I do now. I don't want this divorce, she does. I want to raise my family together but she has other plans. I feel like I need to get custody of my children or she will bring them down with her.
What chances do I have of gaining custody?
What are the first steps I need to take?
I am lost and confused and depressed, and I don't want to put my kids through this.

You would need to see an attorney and file for custody.

You need to be advised though, you should find a job asap even if it is not in the field you normally work. Not having a job is going to hinder custody. You also are probably not going to be able to move the kids out of state.

Some family courts offer mediation, see if your wife is willing to do that if you 2 can agree on custody, visitation and child support.

It is hard for men to get sole but not impossible. So maybe you can see if she will agree to joint?

You might qualify for legal aid, you can call them.

How long has she been living back in Michigan? You should have stayed working in Florida and filed in Florida for the courts to bring the children back to Florida. If she hasn't been living in Michigan for 6 months, nothing can be done until you are both legal residence of Michigan. If it has been 6 months or longer then like Duranie said above, you will have to have a stable job before even getting visitation. I have seen on this site where the courts hold off visitation rights until the noncustodial parent has a job. The courts probably wont give you custody over the mother unless she has been proven and found unfit.