The Hague is easily reachable by public transport. The nearest airport is Rotterdam-The Hague, although you might find there are more flights available to the Netherlands' main airport, Schiphol. From Schiphol, it will take about 30 minutes to get to the Hague by train.
For information on train travel within the Netherlands, use Google Maps or the Dutch rail provider NS (https://www.ns.nl/en). The Hague Central Station (in Dutch: Den Haag Centraal) is 5 minutes' walk from the station.