I am trying to calculate, then reclassify, a DEM raster to show areas that are 20%slope and greater. I am able to calculate the slope using the slope tool in Spatial Analyst, but I am having trouble interpreting the output raster. The output raster has extremely high values and it doesn't look like the result I could then just reclassify to show 20% and greater. Am I going about this in the right fashion? Does the Z factor have something to do with this?

I'm guessing this DEM is in a geographic coordinate reference system (decimal degrees), horizontally, and meters or feet vertically. You must first use the Project tool to switch to a projected coordinate system (preferably in the same horizontal units as vertical elevation), then run Slope to get meaningful values.