my ipod touch 4th generation

I have recently got a new laptop and downloaded itunes for my ipod, but when i plug my ipod in to put new music on it syncs but no music syncs on to it. When i went on to music in the top bar it said sync music and i ticked the box but it says it will delete all my music from my ipod thats already on andput my new music on.

How do i put my new music on without deleting my old stuff??????