Do you use MobileMe? Should happen automatically if you have the accounts set up on both machines, but if it doesn't check your MobileMe sync preferences on the iMac.
Either of those is superior to syncing through iTunes. But if that is what you need to do then connect the iPad, go into iTunes, select the iPad, go to Info tab and set up syncing for your contacts there. The contacts will only sync when you sync the iPad to iTunes.