Go on the court's website and see what forms they have available. Generally most domestic relations courts make pre-printed forms available that are very easy to use. In Ohio the procedure you're looking for is called Modification or Reallocation of Parental Rights and Responsibilities. Ask the clerk what form you need to fill out to accomplish this. While you'll be 100% times better off with a lawyer, you can, in theory, argue this motion on your own. I urge you to read the part of the code in your state that deals with custody. In most states there is a presumption that a fit parents has a right to visitation with his child. If you don't have a criminal record or history of abuse / neglect, you have a good shot at winning some visitation. Be sure to document every time your ex refuses to let you see your child.
By the way, what did your divorce decree say? If it says you have visitation, she cannot bar you from seeing your child regardless of whether your child support was ever late, whether you've missed days in the past, or whether she just feels like the child is better off without you. None of these are reasons to violate a court order. If she is doing this than she can be held in contempt.
Also in some states like Ohio, a party can ask the judge to order the other party to pay for that party's attorney fees. If your ex wife is illegally keeping you from seeing your child in violation of a court order you might have that option if its allowed under PA law.