You will need (for a small one-person omelette):
one big egg
some milk (2 or 3 tablespoons) (if you think you added too much, just add a bit of flour)
a pinch of salt
toppings of choice (I usually add cheese, onions, chives, paprika and some ketchup)
Just mix the ingredients together with a fork and pour it onto a pre-heated frying pan with a bit of oil on it. When you omelette doesn't stick to the pan anymore flip it to the other side and put the toppings on. Wait few minutes until the cheese melts, fold it in half and eat it.
The whole preparation takes only about 10 minutes.