Ступня - is the surfase of you foot facing the ground when you stand/walk.
Нога - the whole leg from ступни up to where your legs begin)
Also нога may meen 'leg up to the knee' or 'up to the ancle' (стопа) - sinse we have no separate word for 'foot'. Size of boots (размер ботинок) in Russian corresponds to 'размер ноги' (feet size) or rarely 'стопы'.