An excellent resource when considering graduate school in mathematics or statistics is a faculty advisor in your area of interest. Talk with your faculty advisor about attending graduate school.

IUPUI Graduate Office
IUPUI's Career Center Graduate School Page
Purdue's Center for Career Opportunities Graduate School Page
The Graduate Record Exam (GRE)

An annual publication from the American Mathematical Society (AMS) with profiles of math departments. Also includes an overview of the job market for Ph.D. mathematicians. (This can take several minutes to load.)
Assistantships and Graduate Fellowships in the Mathematical Sciences

Lists of U.S. Doctoral Departments and Programs
How to Find Money for Graduate School - Internet Sources of Financial Support
Thomson Peterson's Graduate Planner
Gradschools.com