Move your content to a new Mac - Apple Support describes how to do it. The easiest way is wirelessly — if your old Mac connects to the Internet over Wi-Fi, then it will work.
The best time to transfer content is when you first turn on and set up the new Mac. If you declined that step when it appeared, that's ok, but if you do it later you will wind up with two User Accounts. That's still ok, but you will probably want to delete the first one, which probably bears the name you prefer. Write back if that's where you find yourself.
Transferring information can take a long time; the better part of a day is not at all unusual.
As that document explains, Migration Assistant is in the Utilities folder within your Applications folder. On the old Mac, click the Go menu > Utilities. That will open the Utilities folder directly, where you will find the Migration Assistant app.
Lastly, if this is all too complicated you can always contact Apple and have them walk you through those steps. That service is part of the purchase price of a new Mac, as long as you take advantage of it within the first 90 days of ownership. Click Get Support when you're ready.