If your coordinates are in decimal degrees, you would define it as a Geographic Coordinate System with some datum, depending where you got it. Lets say a GCS WGS84
Define Projection—Help | ArcGIS for Desktop
Then you need to pick an appropriate projection for your area... like a UTM, Albers, Lambert (1000s more) or even the web mercator projection (yuck) and project it using the Project tool
for vector Project—Help | ArcGIS for Desktop
for raster Project Raster—Help | ArcGIS for Desktop
Then your quantities/ unit area will have a standard common basis... like units per meter squared, units per kilometer squared or however you want to factor the units/area term. units/square degree is non-sensical since 1 square degree near the north pole is extremely small, compared to the same square degree at the equator,