If you used iTunes:
Open iTunes (without your iPad connected), open Preferences and go to the Devices tab. Turn off auto-sync. This will give you more control of when your iPad syncs with iTunes.
Connect your iPad. In iTunes, select your iPad under the Devices list on the left. Click on the Music link at the top of the page. Change your settings to Selected playlists. . . (don't check the automatically fill free spaces with songs, obviously)
Now be selective about what music you want to sync. There are many options, and you can use them in combination. Keep an eye on the Capacity bar at the bottom to see how much space you have left.
When you've got things the way you want click on Sync or Apply.