All songs purchased with iTunes are automatically in "iCloud".
There is no way to 'push' a song or video to the cloud.
Basically what happens is iTunes now allows you to be able to download any past purchases from iTunes as many times as you want so you don't have to worry about storage space on your local device.
If you don't have a song on your device or tv show or movie, you can go into the iTunes App, Select More on bottom right corner, Select Purchased, Find the item, and hit the Cloud button to download.
Ahh... If you're using iTunes Match things are a little different make sure it's enabled on your new device:http://support.apple.com/kb/HT4913