"The top of the screen" is called the menu bar. It changes, and is based on whichever application is currently selected.
In Mail.app, the attach icon is the paperclip icon that shows in the default icon set of the so-called compse window; when you have requested the creation of a new mail message, or the creation of a reply. In the compose window with the default icons, the attach button is the middle of the five leftmost buttons in the tool bar.
It's quite possible to hide the toolbar entirely (that's controlled from the same pop-up menu as the Customize Toolbar... or can be controlled from the View > Show Toolbar or View > Hide Toolbar within the Mail.app menubar.
To capture an image of the toolbar state (if it's not that the toolbar is hidden, or has been customized), then open Mail.app, create a new (empty) message, and to capture (just) the application display window, press the Command(⌘)-Shift-4 keys as a chord, the cursor will change to a reticule (a cross-hair), place the reticule over the mail application compose window, press the space bar (to highlight the window) and click. This will save a screen shot of just the specified application window into a Screen Shot file (the rest of the name will be the date and time of the screen shot) on your desktop.
Inferring from your references and the terms you are using here, it would appear you might be unfamiliar with OS X and -- if that's the case -- then there's the available Apple Mac 101 introduction that might help clarify the environment.