How to download music from iTunes to my iPhone4s?

Okay so before I was able to highlight the song and drag it to my iPhone file thing. My laptop had issues recently where it had to be cleared out and rebooted so all the programs, files, pictures, documents were erased on my laptop. Afterwards I re-installed iTunes and downloaded two new songs and it won't sync onto my iPhone file! It just kicks it out when I drag it onto the iPhone file. Help me what should I do? I heard it was program to sync music on one computer but it's the same computer! Please help and thanks in advance.😕