Firstly, check if your brake lights come on. If you're on your own (reverse the car up to a wall / garage door) to see if the lights come on.
If they do, it could be a fuse.
If they don't, then it could be the brake pedal switch that detects when the brakes are on or off. You can only shift out of park, if the foot is on the brake, so no brake lights indicates no detection. Grab a torch, and stick your head up under the dash. Locate the switch and twist the assembly at the back of the bracket pushing down towards the brake pedal and it should twist out. Leave it connected to the wire. With it out, check that the brake lights now come on.
If the lights DON'T turn on then switch could be faulty and needs replacement.
If the lights DO turn on then depress the switch fully and check that they turn off - the switch is fine and simply needs to be replaced but adjusted. To adjust, insert back into the hole and push down against the brake pedal arm (depressing the switch) and lock into position. Check that the brake light come on when you put your foot on the brakes.
This was how I fixed my 2008 Holden Epica Diesel.
Town driving: 9.1-9.4 l/100km
Country driving: 6.3-6.5 l/100km