To change how your iPhone syncs with iTunes 12, plug the phone into your computer with the USB cable and click the iPhone icon in the iTunes menu bar. Look through each tab in the iPhone's settings for options about what files to sync, when to sync and how the phone backs up your data.
The Summary tab lists your phone's basic information and system version, and contains backup and other options.
The options in the Backups section affect whether your phone backs up to iTunes when you sync it, or backs up to iCloud once a day, while charging and connected to Wi-Fi. No matter which automatic backup option you pick, click Back Up Now to create a backup in iTunes at any time.
The Options section has controls for several aspects of iPhone syncing. Automatically sync... causes your phone to sync every time you plug it in to your computer. Without this option selected, you have to click the iPhone icon and click Sync manually. Sync ... over Wi-Fi turns on wireless syncing. With this option, your phone appears in iTunes any time it's charging and connected to Wi-Fi, even if you don't have it plugged in to your computer.
Several other options control media syncing, such as only syncing songs that you check in your library, and lowering the quality on songs so that more fit on your phone. Check the last box, Manually manage music and videos if you want to drag and drop media files from your iTunes library to your iPhone. In the library, start dragging any file to pop up a sidebar that shows connected devices. Drop the file onto your iPhone to copy it.
The various data tabs -- Apps, Music, Movies and so on -- have options for controlling which files sync to your iPhone. On each tab, choose whether you want to sync that media type (check Sync Music, for example), and then pick the specific items to sync. ITunes can sync specific playlists or albums, for example, or limit TV and podcast syncing to unplayed episodes.
In addition to picking apps to sync, the Apps tab also has a section for sharing files on your computer with compatible iPhone apps. Select an app and press Add File to send a new document to the app. Press Save To to copy a document from your phone to your computer.
Despite its name, the Info tab actually controls calendar, email and note syncing. In addition to picking a computer program -- such as Outlook -- to sync with, this tab has options to overwrite your phone's data with data from the selected computer program.