Help with using a transistor for a beginner

I am using an Arduino and I want to power an external device.  As I understand it the most I can output from the Arduino is 5v and 500ma.  I need to increase this to 12v and 5a.  It has been suggested that I can do this by running through a transistor.

Can anyone please advise if this is correct and what transistor I should use for this application.

Thanks, Daniel.

[original story: Circuit Digest]

Circuit Digest 10 May 19:02