Often the fist step in a GIS project is to choose appropriate base maps and reference datasets. Search the resources below for finding base maps and reference datasets.
Identify what you are in need of, and what format this data might come in. Do you need administrative boundaries or roads (vector data), or are you searching for a grid of continuous field data like rainfall across a given area (raster data)?
It will help to determine the extent of your study area before you begin. Many state or municipal governments in the U.S. have geospatial data clearinghouses.
Ask yourself who would care enough to create or collect this data? In most cases, the answer is a government agency or a researcher of some type. Think about what organization might be interested in collecting the data (example: glacier data=USGS, National Snow and Ice Data Center).
Don't forget to check researchers' references for where they obtained their data or if their data is available to other researchers. It can never hurt to send that person an email. :-)