How to ..
Down at the bottom of the screen on your Mac is a sort of tray with icons for all the applications you use. It's called the Dock. You can re-arrange the icons by dragging them sideways into a new position and remove them by dragging them off high onto the Desktop and dropping them: they will disappear in a puff of smoke making a whooshing sound!
But fear not, nothing has been deleted, these icons are just short cuts — called Aliases — to the actual Application.
To add another Application to the Dock you need to look in your Applications folder (where else?) by clicking on the blue Finder icon to the left of the Dock so a Finder window opens:
This Finder window is where you do all your File Management, moving files and folder around, creating and re-naming them as you see fit. On the left of the Finder window you will see your Applications folder and in there are all the applications installed on your computer. Some are in folders and some just reside alone. You can only drag an Application into the Dock, not it's folder so make sure you find the Application which has a .app file extension, like Preview.app.
If you set up your Finder Preferences according to the page The Finder Preferences then you will already have clicked the Show all filename extensions box so all the applications will have the .app file extension making them easy to identify if you aren't sure.
So find the Application you want to add to the Dock, click and drag it down into the Dock and a space will appear for it as the other icons move out of the way. Release the mouse to drop it there.
As it is so easy to add and remove Applications from the Dock you might like to spend a few minutes arrainging it to your express satisfaction.
You can also move the Dock onto the left or right side of the screen where it will be a vertical bar rather than horizontal, but this also limits the size so I prefer it on the bottom. This moving is done in the Apple menu on the top left hand side of the Toolbar:
In this menu you can also choose to Hide it by clicking Turn Hiding On and make it all swoopy by clicking Turn Magnification On.
With the Dock hiden simply scroll to the bottom of the screen and it will pop up so you can use it and pop down when you have finished. This is very handy because our beloved Dock takes up some screen real estate, particularly on a laptop. Switching on magnification allows you to make the Dock smaller so you can fit more in it, but still see the icons magnified. To enlarge the Dock go across to the right side of it where you will see a divider:
Although the picture doesn't show it, your icon changes from the regular arrow to a up/down icon indicating that you can click and drag the bar up or down; as you do so the Dock changes in size.