As far I know, there are no public transport to go to Phromthep Cape. You will have to hire a taxi or rent a motobike. Some agencies also offer to go there but generally it is part of a one-day program that ends with watching the sunset at Phromthep Cape. There is a small market on the parking lot where you can buy food and drink and some souvenirs.
To go visit Phromthep Cape, you can also book one of the tours above.