The HearIt Control Panel allows you to pick a "hot key", a key combination used to trigger HearIt. The default is control-H. Pick a hot key that is not likely to be used by any application you use. For example, command-S would be a bad choice because when you tried to save a document, you'd speak some text instead!
To hear some text spoken, select it just as like you would if you were going to Copy it to the Clipboard. Then hit the hot key. There may be a pause (particularly if this is the first time you've used HearIt since you started your Mac), and then you will probably hear your selected text spoken aloud. If HearIt was installed when you started your Mac, you can try it right now!
I said you will "probably" hear your text because HearIt can't get the selected text from all applications, just most of them. Nothing bad should happen, you just won't hear anything, or maybe just a beep. Sometimes you just can't use HearIt in a particular situation in an application (e.g. in a modal dialog) but you can use it in other situations (e.g. in a document).
If HearIt can't speak, the reason will be displayed in the message box near the bottom of the Control Panel window. HearIt remembers the problem until the next time you speak, so if you don't have the Control Panel open when an error occurs, you can open it to see what the problem was.
Fredenburg, Tim. (1993). ReadMe for HearIt. Electronic Document.