I'm trying to calculate the lat/long of points using the Q GIS field calculator.Nathan W suggested to First, save the layer as WGS84 and import it again. Then in the field calculator you can use $x or $y as a variable to get the x and y. but the problem is this: i dont know how to write a expression in field calculator for finding $X and $Y?
You have your answer already. In field calculator expression, just type '$x' (without the quotes) to populate a Longitude field, and the '$y' for Latitude Field. You calculate them one at a time.
If you don't have a latitude and longitude fields yet, you can select the "Create a new field" box on the top of the dialog and create them.
Hope it helps!