You can see the times of ferry services between Santa Cruz de La Palma - San Sebastian de la Gomera in the list that appears when you make the relevant search.
Santa Cruz de La Palma - San Sebastian de la Gomera ferry services to San Sebastian de la Gomera island. So you need to go to port Santa Cruz de La Palma.
There is a limit to the number of seats. It will not allow you to buy the ferry ticket when the limit is already full. Car ferries have an average of 800 people and 200 vehicles and 1200 passengers and 225 vehicles.