I switched computers and my iPhone won't sync

I don't want to brag but I just got a MacBook Pro for my birthday. I used to use windows, the latest version but now when I connect my iPhone to the Mac, iTunes won't let me sync my song into it.

Apple iPhone syncs only to one computer at one time. This means that your iPhone is currently in sync with your Windows computer. To sync your iPhone with your mac, you will have to:
1. Music & Videos - Transfer all music and videos on your iPhone to your Mac (iTunes on Windows will be having the Music you need to transfer, or else you can use a software called Senuti to transfer Media (up to 1000 songs free) directly from iPhone). Only to be done if you have downloaded songs from some source other than iTunes Store (most people do).
2. Documents & eBooks - They will sync via iCloud (easy way, just download them again on your iPhone), or you can choose to copy them directly to your mac from the PC.
3. Apps - Firstly, On your mac, Go to iTunes>Store>Authorise Account (I exactly don't remember the words), and authorise your Apple ID. Then connect your iPhone. If you are using iTunes 11, go to the Devices tab, select your iPhone, and choose transfer purchases.iTunes 10 user, right click your iPhone in the sidebar and choose transfer purchases. This will copy all apps, music, and videos (only if all of them are purchased or downloaded from iTunes).
Your iPhone is now set to sync usually with your Mac. Now you will not be able to sync this iPhone with your PC, unless you repeat the process with the destination as the Windows PC.

You have to log in to your accunt if you have already change the computer