There are many sources of information--faculty, students, publications, and the Internet, to name a few. Once you have decided which characteristics of a program are most important to you, you can start looking.
If you are thinking of going on to graduate school right after finishing your current bachelor's degree program, most of the advice I have encounter suggests that you begin looking in your junior year, so as to have the senior year available for the application process: gathering persons to write recommendations, doing test preparation, etc.
If you are coming back to school after several years, you want to begin the search process well in advance of your intended starting date. You may find, of course, that the time required to do your research and preparation for the admissions process will affect your starting date.
Discipline-specific listings of websites are available on the World Wide Web to help you locate colleges and universities offering degree programs in your field of study.
Another good suggestion I heard on Usenet was to locate the people who are writing your college textbooks. If your department thinks enough of the authors to select their textbooks, those authors might be worth considering as prospective graduate advisors.
For a perspective on determining whether or not you would like to work with a particular faculty member as an advisor, read Professor Marshall Dermer's "An Insider's Guide to Choosing a Graduate Advisor and Research Project in the Laboratory Sciences," available as part of the web page for the UW-Milwaukee Department of Psychology. Many of Professor Dermer's suggestions on choosing an advisor can be applied to study in other fields.
Also ask students studying at the institutions you are considering for their opinion of the program and other aspects of graduate life. Usenet Newsgroups are another good way to locate current students at a particular program in order to ask them about a particular program. Some folks have even put up web pages about their experiences.
You should also talk to people working in your chosen field, as opportunities arise.
There are also a number of books that talk about the what, the why and the how of graduate education. One that I can recommend is The Graduate School Handbook by Richard Jerrard and Margot Jerrard, published by Perigee/Berkley Publishing Group (part of Penguin Putnam).
Click here for some ideas on what the application process is like.
Updated 05122000