Cant get songs to my iphone

When you have your iPhone plugged into your computer, navigate to the Music tab on the left as displayed below. Click Sync Music as you see checked at the top of the window, and then your selected playlists. After selecting all of your desired music to sync, click the Apple button in the lower right corner.

Please make sure that you follow the steps for syncing your music as detailed here.

Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB - Apple Support