The primary ways to reach Port Townsend are by car or by ferry. Whichever way you choose, Port Townsend is roughly 1.5 - 2 hours from Seattle.
From Interstate 5, take Highway 101/12 toward Aberdeen/Shelton. Proceed north on Highway 101 toward Shelton. Stay on 101 and continue north to State Route (SR) 20. Head north on SR20, which will take you right into Port Townsend (approximately 14 miles).
Head west on State Route (SR) 16, crossing The Tacoma Narrows Bridge and proceeding towards Bremerton. Take SR3 north and follow the signs to the Hood Canal Bridge. After crossing that bridge, stay on SR3. When you reach SR19 (Valley Rd.), proceed north towards Port Townsend. When you reach SR20, turn onto it and continue north; SR20 will take you right into Port Townsend (approximately 9 miles).
These directions provide the basic route - always be sure to bring an up-to-date map.