A client of mine violated their contract and is refusing to pay and is avoiding my phone calls. How can I handle this in court?

If the amount you are owed is $13,000 or less you can file with the local magistrate provided that the person to whom the money is owed isn't a corporation or LLC.  If its the latter, then you must retain counsel for matters in excess of $3,500.  If the amount is above $13,000 you'd have to file in the court of common pleas.  Most counties have another max damage limit around $25,000 - $50,000 under which the matter goes to arbitration, which is less formal and faster than normal civil court.