I just got a new MBAir and I am trying to synch my iPhone 3G to it and it won't synch.

Do I have to completely erase my phone or something to synch it?

Also, I have had 5 Macs in the last 5 years (kept upgrading) and now when I try to play some songs, it says I exceeded the 5 max machines I could connect to. I wiped all of those out when I sold them and I just deauthorized my one I just replaced. How can I solve this problem!?

Any advice would be greatly appreciated!