Three magic words that unlock much of the potential on your computer – select, then, do. It doesn’t matter if you are using a mouse, keyboard or even your own voice.
You need to indicate to the computer, i.e. select the thing on your computer you wish to do stuff with.
“Then” is necessary as a pause while you think of what you wish to do or locate the function on your computer.
And, just “do” it. Which for most of us, means invoking a function that actually performs some action straight away. Other functions display a space that allows us to provide further information to the computer, often referred to as a dialogue window or box.
Hint: On many computers, those functions or actions that display dialog windows are indicated by “…” after the name of the function. For example, see Open in the File menu below.

Remember, “select, then, do”. Select the file that you wish to display on the screen, then click the Open button.
Works every time.
There are many ways of selecting things, whether they are functions on a menu, text or pictures in a wordprocessing document, or areas within a digital photograph.
Amongst the myriad of ways that you can select objects, having explored them, you will settle down to a few that you prefer. One that I think is very useful if not universal is “double-click”. You can double-click to select a continuous string of characters. For example, a word or a web address in the Address field of your browser.

Web addresses can get very long and are not fully displayed in the Address field. Simply point to the web address and double-click to select. The “do” part is up to you. If you simply start typing you will replace the selected web address with the one you enter.
Lawrence Goetz has provided a Practice movement, Single clicking, and double clicking exercise that rewards you with a picture of a butterfly when you double-click.
Three magic words – select, then, do.