Q: What will happen if I sync my iPhone that has never been synced to iTunes before?

It's pretty simple and you shouldn't lose any data at all.

Sync your iPhone, iPad, or iPod touch with iTunes using USB - Apple Support

That said, you should always have a backup. Anything can happen at anytime.

fender.bender wrote:

 

I've had my iPhone 6 for about a year now. I just got a new computer and I want to sync it to iTunes from the new computer. I have never synced my iPhone to iTunes before and it has all of my pictures, contacts, and apps on it. I was wondering if there was a way to sync my iPhone to my iTunes account without losing any of my data? Thanks for the help!

That depends on how you have backed up your phone in the past. Have you backed it up to iCloud? Have you enabled Contacts in Settings/iCloud? Have you enabled iCloud Photos? I gather you have never copied photos t your computer, because you do that when you sync with iTunes, and you said you haven't done that.

For now, go to Settings/iCloud, tap on Backup, and back up the phone.

On the computer install the latest version of iTunes (12.3) from http://apple.com/itunes. Then open iTunes, go to Preferences from the menu, Devices tab, and check the box to NOT sync devices. Exit from Preferences. Next, go to the Store menu, and log in using the same Apple ID you used on the iPhone. Go to the Store menu again and click "Check for available downloads" and wait for it to finish.

Connect the phone using a USB cable to a port directly on the computer (not a hub). It should appear as an icon in iTunes. You should also be prompted to import photos; do it. After dealing with photos click on the phone icon in iTunes. You should see a Summary screen. Click Back Up iPhone. If prompted to Transfer Purchases accept it. When everything finished click Sync.

From here forward back up your phone regularly. See: About backups in iCloud and iTunes - Apple Support