At the Last Supper Jesus gave the apostles two great gifts. He gave them the Holy Mass and then ordained them all as priests. On Easter Sunday night He gave them the power to forgive sins in his name.
By making the apostles priests, Our Lord gave them the power to say Mass and give to us the holy Sacraments of the Church.
With the Sacrament of Holy Orders these priestly powers are passed on to other men, making them priests. Only a bishop can give the Sacrament of Ordination. The apostles ordained more bishops to carry on with their work. These bishops in turn ordained more priests and bishops. So it can be seen that the power of the priesthood today comes straight from Jesus Christ Himself.