re:
Without knowing the type of vehicle you have or your technical expertise, I can only summarize for you. Furthermore, I am *assuming* that your use of the term "sync" in this regard means "connect to".
1. On the iPhone, go to Settings - Bluetooth and then enable "Bluetooth".
2. Via your car, initiate the hands-free feature.
3. Depending on your car, the system may speak the instructions to you, and if so, follow them to the letter.
4. Depending on your car, the iPhone or the name associated with your iPhone will display on the system's display with a pairing number.
5. On the iPhone, you should see the model name of your car or hands-free system.
6. The iPhone may automatically input the pairing numbers and prompt you to select to connect to your car or it may prompt you to enter the 4 digit pairing code to connect.
7. Done.