Who founded New Amsterdam?

If you mean the founding nation then;

New Amsterdam was a 17th-century Dutch settlement established at the southern tip of Manhattan Island, which served as the seat of the colonial government in New Netherland territory.
It was renamed New York on September 8, 1664, in honor of the then Duke of York (later King James II of England) after English forces seized control of Manhattan Island, along with the rest of the Dutch colony.

If you mean an individual then;

Dominican trader Juan Rodriguez born in Santo Domingo of Portuguese and African descent, arrived in Manhattan during the winter of 1613–1614, trapping for beaver pelts and trading with the local population as a representative of the Dutch. He was the first recorded non-Native American inhabitant of what would eventually become New York City.