Backing up your data is essential to insure against damage or loss to your iPhone. The process will also make it possible to easily transfer data from your device to a new iPhone should you chose to upgrade.
To write a current backup to your computer:
1. Connect the iPhone to the computer with a USB cable.
2. Open iTunes.
3. Click on your iPhone under DEVICES in the left menu.
4. Make sure the Summary tab at the top of iTunes is selected.
5. Under the Backup section, check the "Back up to this computer" radio button.
6. Click the Sync button.