How to sync my iPhone/iTunes with my new laptop?

I purchased a new laptop because my old one crashed. I also have an iPhone 4.Me and my sister share the iTunes that were on my old computer, and I made the stupid mistake of trying to sync mine with the new laptop, which resulted in me losing all of my songs and apps. (my pics and contacts weren't affected). I would like to try to take the songs that my sister has on her phone and put them back into our iTunes library. How would I do this?

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