One way to educate yourself about the candidates and issues is to watch the televised debates.
September 26, 2008: Presidential debate with domestic policy focus, University of Mississippi, Oxford, MS October 2, 2008: Vice Presidential debate, Washington University, St. Louis, MO October 7, 2008: Presidential debate in a town hall format, Belmont University, Nashville, TN October 15, 2008: Presidential debate with foreign policy focus, Hofstra University, Hempstead, NY
If you're not sure where you yourself stand on the issues, then it would naturally be harder for you to decide who you agree with more. There are a number of quizzes and polls on the Internet - here's one that tells you the candidates' stances on the issues -
Another thing to consider is what type of voter you are. Some people base their political opinions on what they feel is in their own best interest, and others on what they feel is best for the country or the public at large. (Their opinions and reality may be very different, but this concerns opinions.)
I find these different types of voters all over the continuum, from far left/progressive to far right/conservative. There are more than a dozen political parties in the U.S., and it's impossible and unfair to say that people over there vote their self-interest while those over here vote for the public interest.
My feeling on the matter is that it's more responsible to vote for what you perceive as the country's best interest, or in other words, what you believe is best for the majority. Chances are you will personally benefit from living in a society that is stronger, healthier, and happier!