Syncing iPad with iPhone
Make sure you have all of your iPhone songs already loaded in your iTunes library. If you previously synced iTunes on your computer with your iPhone, this is most certainly the case. 
Connect your iPad to your computer and run iTunes
In your iPad from within iTunes, navigate to the "Music" tab and make sure that the "Sync music" and "Entire library" options are ticked. 
Click on the Apply button. 


Your iTunes library songs will now be pushed to your iPad. 

You can alternatively enable iTunes match on both your iPhone and your iPad, and have iTunes Match push your iPhone songs to your iPad. More information about iTunes Match here:

http://support.apple.com/kb/ht4914