Horrible Screeching Sound from Breaks

Likely brake wear indicator on your pads, they are telling you they are worn. Be sure to take it to someone who has had experience in working with the Prius. The brakes are no different than any other vehicle, but there are certain procedures to follow if they want to successfully complete the job.

Here is a good thread for your reading pleasure if you are going to diy or have someone you know diy.
HOW TO : Replace Front Brake Pads and Discs (Rotors) Prius Gen 2 | PriusChat

Also, have your rear drums checked too, they may also need the shoes replace.

 

+1 - brake pads. Bleeding the system is the trickiest part.

The mechanical brakes only engage at less than 15 mph and regenerative breaking above that (unless you hit the brakes hard or are in "B" mode) so that is an easy way to diagnose the systems.

 

Ditto all the above. If your car has over 100K miles and has never had brake service, you may consider getting the brake fluid flushed as well. The brake actuator is very expensive and old brake fluid may cause an early failure. The hydraulic system on the Prius is way different than most and the mechanic needs special tools and skills to flush the fluid, as mentioned above.