Will iTunes delete all of my stuff?

I wanted to upload about 50 songs and ringtones onto my iPhone 4S just now, and when I click 'sync' a message comes up saying, "The iPhone is synced with another iTunes library. Do you want to erase this iPhone and sync with this iTunes library?" I want to put my songs and tones on my phone, but I don't want to have my pictures, contacts, apps, etc.erased. Is this what is going to happen? Or will just my music ALREADY on my phone (about 3 songs) be erased?

From my knowledge, your phone WILL BE ERASED and be replace with your RECENT BACKUP.
If you want to keep EVERYTHING, you should first backup your phone. After you have the backup, then you are able to upload 50 songs WITHOUT LOSING EVERYTHING (Apple will first erase everything, then insert the songs, and use the backup.)

I'm NOT SURE ABOUT THIS, BUT I USE THIS Way ABOVE TO GUARANTEE YOUR PHONE WILL BE THE SAME AS BEFORE.