The ricotta is an Italian cheese made by cooking the whey (liquid byproduct of the cheese production), by cooking it at an high temperature (80-90°C), hence the name "ricotta", which means "cooked again". Even though it's typically made from the whey, you can make it directly from the milk as well.
Pour the milk (and/or whey) and the salt into a pot, stir it well, and heat it up to 80°C.
Once the temperature has been reached, add the citric acid (or lemon juice) and stir well. You should already see the citric acid react with the milk/whey.