House is leaking during heavy rains from exterior walls at bottom where foundation meets floor slab

It's poured not cinder block. But when we get heavy heavy rain it's coming in from certain areas in the corners not completely horrible but enough to definitely cause mold. I've been told I should throw dirt in front to stop/build a French drain etc etc I want the cheapest and most reliable option. Also this is a rental I cant have my renters house flooding. Any ideas