how can I capture my screen from my iMac?

The explanation is long, but its simple to use.

Mac has several options:

COPY TO A DESKTOP FILE:
To Capture Entire Desktop
• Switch to the screen that you want to capture
• Press and hold the Cloverleaf/Command (?) key next to the space bar, the Shift key, and the number 3 key (?+SHIFT+3) and release all keys.
• With your mouse, click on the screen
This takes a screen shot of the current screen and saves it to the Desktop as a pdf/png file.

To Capture a Portion of your desktop
• Switch to the screen that you want to do a segment capture
• Press and hold the Cloverleaf/Command (?) key next to the space bar, the Shift key, and the number 4 key (?+SHIFT+4) and release all keys.
• The mouse cursor will change from an arrow into cross hairs +. Simply click and drag the cross hairs over the area of the screen of which you want to take a picture. When you release the mouse button, you will hear a clicking sound reminiscent of a camera shutter and a new file will appear on your desktop in pdf/png format.

To capture an application window:
• Switch to the screen that you want to do a segment capture
• Press and hold the Cloverleaf/Command (?) key next to the space bar, the Shift key, and the number 4 key (?+SHIFT+4) and release all keys.
• The mouse cursor will change from an arrow into crosshairs +.
• Press the space bar once.
The mouse cursor will change into a camera. Move the camera to the application window you want to capture and press Enter. A new file will appear on your desktop in pdf/png format.

SUMMARY:
?+SHIFT+3 ==> Copy desktop
?+SHIFT+4 then select by dragging out box ==> Copy desktop region/selection
?+SHIFT+4 then hit spacebar and click on window ==> Copy window

TIP: TO COPY TO CLIPBOARD
If you add the control key to any of the above screenshot keyboard shortcuts, the screenshot is copied to the clipboard instead of being saved to a file; you can then paste it into any graphics application and save it in any format you prefer.
?+SHIFT+CTRL+3 ==> Copy desktop
?+SHIFT+CTRL+4 then select by dragging out box ==> Copy desktop region/selection
?+SHIFT+CTRL+4 then hit spacebar and click on window ==> Copy window

TIP: Using Grab to take Screenshots
The Grab utility is located in the Applications -> Utilities folder. To use Grab, double-click on its icon and choose the type of image you want to capture.
http://www.wikihow.com/Take-Screen-Shot-or-Print-Screen-on-a-Mac-Using-Grab

There's a program built into mac called "grab" use this

I will recommand a software which might be useful:
Onde Screen Capture for Mac is a professional and powerful screen grabbing utility running on Mac OS
ondesoft

press "command+shift+4" together, then use you mouse capture what you want