Your attorney general's office should be able to COLLECT child support for you, but you must have a court order requiring it. If you do not already have a parenting plan in place, you will need some court order.
Try your local legal aid group (www.philalegal.org) or your county bar for free assistance in obtaining an order. Otherwise, the court clerk can provide you with the proper forms. You can start with a paternity action. Normally, filing a case with the court costs money, a filing fee. However, the court may waive the filing fee if you are "indigent." There is another little form for details about your income. From this form, the court can decide whether you are indigent.
If you would rather avoid the courts, but the father of your child is not offering to pay support, may I suggest a neighborhood mediation group. Try phillymediators.org. This site says "no one is turned away for inability to pay." The two of you can work something out in mediation without going to court. It's cheaper, friendlier, and more flexible.
Go to the family court in Philadelphia and file for support for the child. They have a help desk there and you will need to fill out the paperwork and they will then set this up for a conference which you will need to attend. If the father works, it will be wage attached. If you are getting assistance from welfare, they will have a lien on the funds.