Congratulations on the new iMac. To transfer the files on your MBA to that machine, when you FIRST boot it, it will go through the "welcome" stuff and then offer to migrate your data. You definitely WANT to migrate your data, so don't pass up that opportunity. If you do, the very next thing it will do is offer to create a new account for you on the iMac, and if you do that, things get a bit more complicated.
Now, before the new iMac is powered up, you need to prep the MBA to make the transfer and connect the two systems. There are a couple of ways. Your best bet is to read this:
https://support.apple.com/en-us/HT204350and use one of the connection processes listed there.
But note this sentence at the end of that article:
After Migration Assistant is done, log in to the migrated account on your new Mac to see its files.
What that said is that if you do what it says, you will end up with a NEW account where the data from the MBA has been migrated. That's why I said to stop and accept the offer of a migration that comes on the new iMac BEFORE you create a new account. If you already have created an account, Migration Assistant will not write files into that account, but create a new one for the migrated data or completely overwrite the account if you tell it to do so, wiping out anything already there. If you let MA run before you create an account on the new iMac, then it will create an account for you with exactly the same name and login as on the MBA, which is pretty useful.
And of course, backup the MBA before you start. Then test the backup. Mucking about with full file migrations is a major activity and backup before undertaking it is a very good thing to do.
