How do you update an iTouch on a new computer without deleting everything on it?

How can I sync my itouch and my new mac without deleting all the songs, apps, videos, etc. on my itouch? Before my mac, I had a pc that I had connected with my itouch. The pc had all the songs and apps and stuff. The new mac has nothing and I'm afraid that when I try to update my itouch by adding a new song or app from iTunes from my mac, all the old ones on my itouch will get erased. This has happened before so I'm paranoid. I know you can check some options but I'm not sure which ones and I don't want to take the risk without knowing for sure which will simply update my itouch with new media while others will replace it completely. So how would I do this? Thanks!