Changing the Background

That is to say not the background of the entire page, but the background of one particular element.  You can change the bgd. of any element including images, form elements, and tables.
When using a background image:
a. Type background-image:
b Then type url(image.jpg), where image.jpg is the name of the image you want to use for the background, or type none if you don't want to use any image at all ie:
 #wrapper {background-image: url(guitar.jpg);

To repeat a background image:

Type background-repeat: direction, where direction is either repeat (to tile the image both horizontally and vertically), repeat-x (to tile the image horizontally), repeat-y (to tile the image vertically), or no-repeat (to not tile the image at all) ie:  background-repeat: repeat;}
To specify the exact position of the  background image in an element:
Type background-position: x y, where both x and y can be expressed either as a percentage or as an absolute distance. Or use values of left, center, or right for x and top. center, or bottom for y   ie:  background-position: left top;  

To change the background color:
a. Type background-color: 
b. Type transparent (to let the background of the parent element show through) or type color, where color is a color name or even an rgb color.

The following defaults should also be noted: the default background - colour is transparent, the background - image default is none, the background - repeat default is repeat, the background - attachment default is scroll, and finally the background - position default will be top left.

If you want to set a background for the entire webpage then simply set the background property for the body element.  Another point to bear in mind is that when you specify both the URL and the colour for your background, the colour will be used first until the URL is actually loaded and the colour will be seen through any transparent areas of the background image.