You're using two different terminologies here. Desktop is a stand alone computer and it uses programs and plug-ins. Apps are found on mobile devices and devices have home screens.
I'm going to guess that it is a device, only because you ended up here, in which case you probably need to pinch the screen, put both fingers on the screen and move them together, until you see your screen options. You might also get there by long pressing on an empty spot on the screen. Somewhere there it should say apps and widgets. You want widgets. Scroll down until you see the one you want, and then drag it up to the screen you want it on.
Welcome to Android Central! To build on VidJunky's helpful response, I'd also guess that you're asking about the Google search bar widget (which has a microphone symbol on it as well). If the pinch gesture on the screen doesn't get you to the Widgets list, then try instead to long-press any empty part of the homescreen.