Two kinds of menus are available, the Menu bar and Context menus (also called pop-up menus).
The menu baris at the top of the screen. If you click on any word or press [Alt] + the key for the underlined letter, a menu with a number of choices will drop down:
Some choices may be grayed out (like "Search again" in the illustration). These choices are not available. You can choose the others by pressing the key for the underlined letter or by using the arrow keys to move up and down.
Any choice that is followed by leads to another menu. Some choices are followed by a keyboard shortcut (such as Ctrl+F above). You can use these shortcuts instead of the menu.
If you click with the right mouse button at any point on the screen a menu will appear with a list of possible commands. What is on the list depends on where the cursor is.
You can also open a context menu by pressing [Shift]+F10.
Context menus work the same way as the drop down menus from the Menu bar.