About the only possible thing you can do at any sort of a reasonable price is to get a new rear wheel with internal gears (also called an IGH short for internal gear hub) and add brakes if you don't have them.

The bike you bought was designed to be a fixie/single speed and really is not compatible with adding derailler gearing. (width of rear wheel, no attachment point for deraillers, etc)

Better to look for a used bike.

good luck