for Computer Science Graduate Admissions
Choosing a Graduate School in Computer Science - explains all aspects of the application process.
Applying to Ph.D. Programs in Computer Science (pdf) - explains everything in more detail.
The Graduate Student Information Guide (pdf) - among other information at CRA-W, helps with planning an approach to graduate school.
Advice for Undergraduates Considering Graduate School - is a classic essay.
Understanding Your Scores - has a wealth of information on what GRE scores do and do not mean, as does the GRE Guide to the Use of Scores.
Companies exist which specialize in leading students through the application process; some of these are probably very helpful.
But in the end your professors are your best source of advice.