i have replaced my 10 yr old Mac with an airbook but I dont know how to transfer only some of the data. .

It really depends on how much data you need/want to transfer. If it's not that much, then a thumb drive or external drive would work fine.

For transferring settings, apps, etc., then the best method would be via Migration Assistant. This Apple Support article describes it and what it can transfer: Transfer to a new Mac with Migration Assistant - Apple Support

For partial transfers of select data, I always use Finder and an external HDD to get the files I want off the old drive and onto the new drive.

Be aware, it you're trying to transfer 3rd party apps, that may not work. It may not work regardless of transfer method. This is especially true of those apps that are licensed. These should be reinstalled from fresh downloaded media.

If you data is in iCloud, signing in with your existing Apple ID will make them available.