What must I do to my car after i have realised I have filled in brake fluid in the engine?
Do NOT run the engine.
Drain the contaminated engine oil and refill with fresh oil (cheap oil is sufficient here). The idea is to use the cheap oil to rinse off the surfaces inside the engine that have brake fluid on them.
Run the engine for 5 minutes, then stop the engine. Allow to stand for 5 minutes.
Now drain the oil again and refill with fresh oil (quality oil this time), also change the filter this time.
The oil should be fine now, after changing twice and the filter.
Drain the oil from engine. Replace with new oil. Just like doing a normal oil change. Do not run the engine until you change it.
Properly dispose of oil/brake fluid; for example auto parts stores often recycle all old oil together so it's OK to bring the mixed fluids.