Yes, there is a lot to learn so you can clone effectively/efficiently, and there are other tools that are also useful for the same kind of editing, like the Spot Healing Brush, Healing Brush, Patch Tools, the various Selection tools and CS5's masking functions. A very handy CS 5 feature found as part of several tools and functions is Content - Aware.
Adobe TV
Adobe TV
Adobe TV
On the Applications bar you can click on Help > Photoshop Help to access Adobe Community Help which has a variety of resources, but it's even easier to just press the F1 key on your keyboard.
Each tool has an Options bar, and for Cloning there is another handy aid - the Clone Source Window.
When cloning, you can use the selection tools and masking functions to make portions of the image unavailable for editing. For instance, you want to clone away a background object that is partially obscured by a person's arm.
You can select the person's arm using one of the selection tools, and then invert the selection so the arm is no longer an editable part of the image. That way you can clone right up to the edge of the arm.
To invert the selection, up on the Applications bar you can click on Select > Invert (keyboard shortcut: Shift -CTRL-I). a good habit to develop is to save any selections you make (Select > Save Selection) as soon as you have finalized them.
http://tv.adobe.com/watch/photoshop-20th-anniversary/getting-started-03-retouching-blemishes/
An excellent book to have for CS 5 reference is
Adobe Photoshop CS5 for Photographers: A professional image editor's guide to the creative use of Photoshop for the Macintosh and PC