My girlfriend and I have been together (not married) for 14 years. My son's bio mother is not really in the picture (although I have no doubt she would relinquish her rights if asked).
I have an almost 15 year old son. My girlfriend has been my sons "Mom" since he was 3 months old. She is nthe only mother he knows.
Now, my girlfriend would like to adopt my son.
Can she adopt him?
What steps need to be taken?

Get married and then after a years time apply for adoption. Use a lawyer so no steps are skipped.

You left out the one most important detail, where you are. Most areas require that a couple be married for a period before they will approve a "step" parent adoption. But there are a few areas that don't.

Since you need an attorney to prepare the adoption petition, I would start shopping around for one, who can advise you on the local laws and proceed from them. Once you have satisfied the requirements, if the bio mother will be agreeable it's a simple and easy process.