Either party can petition the appropriate court for an adjudication of paternity (if this has not already been established in connection with the issuance of the birth certificate), and for a parental responsibility and child support order, at any time, if no such order is in place (assuming that Pennsylvania is the "home state" of the child and venue is proper).
Child custody and visitation rights are determined based upon the "best interests of the child" with very little other formal guidance from the statute or even case law which also affords a judge very broad, although not unlimited discretion in resolving the issue if the parents don't reach an agreement. Any parent who is not adjudicated to be "unfit" (whose parental rights would then be terminated), is entitled to some reasonable visitation under the circumstances at a minimum. In practice, courts tend to prefer to enter an order that preserves the pre-litigation status quo is one was established for any reasonable length of time.
Once a custody and visitation schedule are established (logically, this is actually done contemporaneously), a child support award is also entered based upon the number of nights per year that the child spends with each parent, the income of each parent, and the extraordinary expenses, if any of the child, pursuant to guidelines that exist under state law but are federally mandated. Generally speaking, child support payments are quite small relative to the incomes of the parties. If a parent is willfully refusing to work or underemployed so as to reduce child support, in some circumstances, income that could have been earned is imputed to that parent for purposes of determining the appropriate amount of child support.
There is a small body of civil procedure that goes to the nitty gritty of how this is handled after a petition is filed, but that really goes beyond the scope of the question.
In the simplest case, only the two parents are involved in the litigation.
But, there are circumstances in which there can be other parties. For example, if the child has been supported by welfare, a representative of the state is a party to make sure that child support is paid reducing the need for welfare payments or repaying welfare payments already made by the state. Guardians of an un-empancipated minor or disabled parent might participate. Grandparents can sometimes have standing to participate. A non-parent who has physical custody of the child would usually have standing to participate. This all gets a little technical, and since the question doesn't suggest any facts that would call for additional parties, I will leave it at that.
In the event of a substantial change of circumstances after a statutory period after the last order was entered, a court may start over, more or less from scratch, and establish a modified child custody, visitation and child support order that reflects the changed circumstances. As a practical matter, for parents who are separated from shortly after the birth of a child and can't work things out between themselves, there will probably be several to half a dozen modification proceedings until the child is an adult.