Okay, I will explain it as simply as I can.
First click anywhere on your desktop that does not have any kind of open window on it, so that the Finder shows in the menubar next to the black Apple, as in the sample image below.
Click on the text that says View Image Full-Size, that will open the image to make it easily viewable.
Once you have that then click on File and select New Finder Window, as it shows below.
Depending on how you have your preferences set to view the finder window you may see something like this.
This next shot shows you where Places are.
Click on the small triangle to reveal different places in your home folder, the one we are looking for is called Applications.
It may also help if you put the finder into column view, this image below shows you how.
Once you have put it into column view this is what you will see.
Yours may look a little different but it will be very similar.
Now click on Applications and scroll until you find the Mail, it will have an icon that looks like this.
Now comes the important part, grab hold of that with your mouse and drag it to the dock, do not let go until the other icons move out of the way to make room for it, if you let go too soon then it will move the application to your desktop and that is not where you want it.
If that happens, then drag it back to the applications folder where it came from and try again. What this does is put an alias, that's the same as a shortcut, into the dock to make it easily accessible.
Now it is back where you want it. You can red dot the finder window to close it.
This link will take you to a website on the Apple site to help you learn more about your Mac and how to use the dock.
If you have any problems please let me know.