If I sync my iPhone to iTunes will I lose any information on my iPhone?

Syncing generally copies data from iTunes on the computer onto the iPhone.

Data can include, Music, Movies, Photos, Tv Shows.

The Syncing process will generally overwrite the iPhone's content in those categories.

If you have Music on your iPhone that does not exist in the iTunes Library on the computer you are syncing to, that music will be removed from the iPhone. Same for Movies and Tv Shows.

In the case of Music, Movies and TV Shows purchased from the iTunes Store, it will tell you there are purchases on the iPhone that have not been transferred to the iTunes Library, and you can transfer them before syncing.

Photos are a bit different. If you Sync Photos to the iPhone it does not overwrite the Photos in your camera roll, but will overwrite any photos that had been previously synced with the new Photo selections.

The Purpose of Syncing is generally to transfer data from an iTunes Library that already holds all the data, to the iPhone.

If you have never synced before, why do you want to do this now? What are you looking to accomplish?

If you just want to backup, you can set iTunes to not Automatically sync with the iPhone so nothing is overwritten.

In the iTunes Menu on your computer if its a Mac or the Edit Menu if its a PC, go to Preferences->Devices, and check "Prevent iPods, iPhones and iPads from syncing automatically".

More details here: Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB - Apple Support

This question pops up once in a while, this discussion might help When I sync, does my iphone override my Mac, or vice versa?

From my experience Music usually syncs Mac to iDevice, Photo’s the other way round, Podcasts get updated on each device ie. iDevice and Mac independently but played or position information can go either from the most recent (this might be cloud based now).

Contacts and Calendar work similar to Podcasts, ie device with latest info goes to other. Saying that, if you have iCloud enabled for Contacts on both then it’s disabled in iTunes, because it’s basically not required.

Hope the above helps a little.

ps. Before doing anything it would be wise to do two things, first make sure you have a BackUp of the iPhone either via Cloud or iTunes, second if you have any music on the Mac go to you Music Media folder and copy to an external drive, that way you have the originals. You should consider similar for any Photo’s, Contact’s and Calendar, but because they can be in the Cloud it might not be so important.