Q: How to transfer files from my existing Mac to a new Mac

During the process of starting up and registering your new Mac (congrats by the way!) you’ll be asked if you want to migrate your data and/or applications to the new computer. If you have been using TimeMachine to back up your computer onto an external hard drive you have the perfect vehicle for moving into the new Mac. Plug the external drive into the new computer, wait for the new Mac to recognize it, and then select the account(s) you want to migrate. If you have a TimeCapsule you’ve been backing the computer up to wirelessly do not attempt to migrate via WiFi because it is too slow. While you can physically connect the computer to the TC with an ethernet cable I don’t recommend it. I’ve experienced too many stalled migrations over ethernet to trust them. This is why I’m not a huge fan of the TC.

You can also connect the two computers via Thunderbolt cable and boot the old computer into target disk mode by holding the T key while you restart the old computer. If the old computer doesn’t have Thunderbolt you can purchase a FireWire->Thunderbolt adaptor. But since TB cables and the adaptor aren’t cheap, if you don’t intend to use a TB device and you don’t have an external drive your best solution is to purchase a USB 3 external drive, back up the old computer and then use it to migrate to the new computer.

Please carefully read Apple's article that explains everything you need to know to accomplish your goal.

Move your content to a new Mac - Apple Support