It all depends on what state your in. I live in NY and Im working on being a legal guardian for my older (handicapped) sister. Its a very long and drawn out process and will cost a good amount of money. We started out by getting paper work from our local county/DSS building, in NY they are right in with the visitation/custody paper work that you can just go and pick up any time. Its involves alot of paper work and signatures by alot of different people then has to be turned into the courts and be approved by a family court judge,you will get a court date in the mail. If your child's father is the birth certificate but not in the child's life, all you have to do is put down his last known information when the paper work asks and if he still lives there,he will get a copy of everything and if he objects he will have to show up to the court date and tell the judge he does not want it happen and why but if he does not show up then that automatically means he agrees or does not care. Hope this helps and good luck!
Talk it over with an attorney that specializes in family law. Whatever you do, you want it to be done correctly. Look into low-cost legal clinics if you can't afford a regular lawyer.