You can get to Herzliya itself by train, cab, or bus.
The train station is located on the lower level of Terminal 3. You can take the train from the airport to Tel Aviv (Savidor Center Station) and then change trains directly to Herzliya.
To get to Herzliya by bus, you need to take bus number 5 to the Airport City station. From there, there are buses to Haifa, and any of them will suit you.
If you are not traveling alone, it is advantageous to take a cab to the resort.