To start an application (software program), click its icon in the dock (if its icon is in the dock), or double click its icon in the folder where it is located. Applications are usually in the Applications folder on your internal Mac hard drive (Macintosh HD disk). Or you can drag a file to an application icon in the dock to open the file with that application.
To switch from one application to another, you can click the other program icon in the dock. Or you can use this keyboard shortcut: While holding the command key down, press the tab key. Icons for all the currently open programs appear, and you can move between them by tapping the tab key.