I am getting ready to create the graphics for my website.
should I design this all based on the 800x600 pixel size
or should I move up to a larger size?

I want to create a background for my website, and not sure what the
most widely used standard size is.

Deanna

Neither do I. It may depend on the location.
http://htmlhelp.com/faq/html/design.html#screen-size

Only you can answer what screen you want to support, but here is an idea of the general public's screen resolution preferences.
Current Screen Resolution stats

If you plan to make a fixed size image which "is" the page, be aware this is a generally poor idea. Make a flexible page, with graphical elements that enhance it, and people on a slow connection with a different sized browser window won't have to watch the roller blind effect.

i dont know how to put a image background and personalize my profile....

Please see the FAQ entry How can I specify background images?