
The Gemological Institute of America is a super reliable source for information. Please read about the 4C's on their site to learn about how the relationship of Color(whiteness), Clarity(number of inclusions), Cut(proportionally, not style) and Carat(overall weight) define the quality and value of a stone.
click here: GIA - About the 4C's
Ok, now that you know the basics, consider which characteristics are most important to you given your budget. Do you want the largest stone possible or would you sacrifice size for a white white diamond? Its a trade-off. As an extreme example, for a similar investment, you can but a .25 carat D color Flawless stone or a full carat M color I3 (very included) diamond. Fortunately, there are also many other choices in the middle.