Hi Christine,
It is great to hear you will soon be visiting Derry!
Most people visiting the Giant's Causeway from Derry take the bus or train (preferred scenic route for tourists) to Coleraine!
At Coleraine Train/Bus station, ask for the 402 Causeway Rambler - It leaves every 30 minutes!
Let me know if you have any questions about your upcoming trip and I would be happy to help,
Cathaoir