The process is called syncing and you can select to synch all of your music or only certain Artists, Albums, Playlists and Genres
Select your device either from
Top right by iTunes Store Button - new look iTunes11
In the lefthand panel - old look as per itunes 10
Select the music page at the top
Tick Sync Music
If you have enough space on iPad to sync all your music select that or if not select music from selected playlists and select away.
The Bar at the bottom will show how much space on your device you are taking up. When you have made your selections click Apply Bottom Right and Sync will Start.
In the new look to get back to the library click the blue Done button Top right.