Welcome to iMore!
First open iTunes and plug in your phone. Once your phone appears there are a couple of things you will need to check and uncheck. I will provide screenshots to help you. Please, if you have any further questions, feel free to become a member and ask away.
1. Under
Backups: Check,
This computer (highlighted red). This will allow your phone to sync to your computer and not iCloud.
2. Under
Options:
Uncheck, Automatically sync when this phone is connected (highlighted yellow). Unless you would prefer that to happen. Then leave it checked.
3. Under
Options: You can check or uncheck,
Manually manage music and videos (highlighted red) depending on how you want to do it. I personally manage it under the Music Tab in the left hand margin as you will see in my second screenshot. I find it just as easy to do it that way. Enough selections there to easily allow you to manage your library.
Hope all this info helps you!