ITunes wants to delete all of my music

Okay so I just got my iPhone 4 and I want to sync it to iTunes with my iPad since iCloud doesn't work. I connected my iPad to my PC with the cord and tired to sync it. But when I tried to sync it, it said:

"Are you sure you want to revome existing music, movies, etc. From this iPad and sync it with this iTunes library?
Music, movies, etc.synced to "<my> iPad" from other iTunes libraries with be removed and items will be synced to this iTunes library."

I'm so scared. What does this mean? If iTunes want to delete all my songs and etc., how can I fix it? Is it possible to also sync my iPhone too?

This means that the device had been synced with a different computer. You can only have one device with one computer (actually only one iTunes library). If you synced with a different computer of library you need to sync with that library. If you do not have access to it than you will need to erase all your contend, register the device with your computer and just re-download anything you lost from the cloud (for free).

"What does this mean? "

Sounds pretty self-explanatory to me. If you sync, any stuff not on your computer will be erased from the device.

That's what sync means - to match one device against another.

This has been a basic feature of iOS devices for ages, yet every year brings millions of new owners who each spend hundreds of dollars on devices they don;t really understand.

"how can I fix it? Is it possible to also sync my iPhone too?"

You don;t fix it, you set for manual management of music and video.

Why would you want to sync your iPhone if you can't sync your iPad? That makes no sense.