qgis - Summing attributes intersected by under a radius

Apologies - new user.

I am working with QGIS on Mac OS X and with shapefiles downloaded from the US Census Bureau that show population data by state and by county.

How do I:

1. select certain points (e.g. cities)


2. draw a defined distance radius around those points (e.g. 500km - At the moment I can only get the units to be in degrees)


3. Sum the populations in the counties that are intersected by the circle?

Any assistance is greatly appreciated!