Example: Effect of minimum wage on unemployment
1) A magazine article reporting on recent studies linking the minimum wage to higher unemployment would be a secondary source
2) A research article proving that minimum wage is linked to unemployment with supporting data collected for that study by the researcher would be a primary source
3) If you are looking at how the media presents the effect of minimum wage on unemployment, newspaper article could be used as a primary source
The exact policy (if you are doing a policy focused paper) you are using to frame your paper would be a primary source, anything written about that policy in terms of analysis, interpretation, conclusions drawn, criticism etc. would be a secondary source.