Calculating polygon areas in QGIS?
Make the layer editable, then use the field calculator (Layer>Open attribute table>Field Calculator/Ctrl+I or right click shapefile>Open attribute table>Field Calculator/Ctrl+I). There is an operator "$area" that will calculate the area of each row in the table. All units will be calculated in the units of the projection, so you probably want to project it to a projection that uses feet or metres before doing that, rather than lat/lon.
In current versions of QGIS if your data is in a Geographic CRS you can use the function transform() to project the geometries to a projected system (Preferably an equal area one) without the need to duplicate your data. In the Field calculator, something like this should work: area($geometry, 'EPSG:4326','EPSG:3763')
@AlexandreNeto: Do you mean `area(transform($geometry, 'EPSG:4326','EPSG:3763'))`?