I can't sync my iphone into my laptop! Urgent

When I first got my mac, i synced my music into it then synced it all onto my iPhone. My mac had everything and now, empty. I can't find any songs or apps. I tried syncing my data onto my computer but it warned me, "This action will replace all data of your Iphone with the computers. The thing is my computer doesn't have anything.
I have no music. No movies. No apps.
What happened and how can I sync my data onto this new computer?

For photos and videos, see the first link below.

For music files, see the other options at links below. Most people use iCloud, since it is free and works like a charm.

If you stick to the Apple idea of using the computer to gather music, and then sync the iPhone to the computer, you will not have to do this backward flow of music going from the Internet to the iPhone and finally to the Mac. If you are getting your music from another computer, just start keeping a thumb drive in your pocket when you visit friends' homes. Copy their music to the thumb drive and then to your computer.

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