I want to transfer my files from my old Mac to my new Mac. What is the first thing I need to do?

Elaborating a bit on the response from razmee, after you have a good reliable and complete backup done, you can use the Migration Assistant to migrate files and applications from your backup drive to the new computer. The best way to accomplish this is on very first boot up of the new computer, respond to the questions and indicate that you will be migrating from an external backup drive.

You can migrate EVERYTHING. Or you can be more selective; for instance you can bring over just user accounts and files, and then reinstall applications from scratch -- the advantage of this is that you will be installing the latest versions and avoid the risk of bringing over an older version that does not work as well with the newer operating system,

Depending on how old your "old Mac" is, it might have some 32-bit applications. The new computers from Apple only run 64-bit applications. So, for example, an old version of MS-Office from 2011 is 32-bit and will need to be replaced with a new installation of a more recent version of MS-Office. Same thing for some older Adobe programs, and other various software.

kathleenfrompine hill wrote:

I had a new Mac delivered today and want to transfer files from my old mac to the new one. What is the first thing I need to do.

Read here Back up your Mac with Time Machine - Apple Support