Making A Countdown Timer

I need to know how to make a countdown timer. I am making a that needs a countdown timer to end the game. The Player collects the objects and need to collect them all to win, but I want to connect this to a timer. The timer needs to countdown 3-2-1 before it allows the player to move. I have looked at some of the old questions but I could not get any of them to work.

My dad does coding in his gaming company he runs so I decided to start early. 13 is early for me. I started using Unity a month ago and I'm still confused on quite a few things. It needs to be in c#. I'm not sure if it needs to be in a new script or if it can go on my player script. I would appreciate any help given.

Then I am trying to connect the time to a end game/retry menu if the timer runs out and a "you win!" with a trie again button, recording the players previous score.

Thanks, I know that this is a big project.

People I found That I almost understood: @jacobschellenberg @DuckWare Games @DeathDev