You should brush your teeth once in the morning, after you've woken up, and once at night, before you sleep. You should make sure you brush for 2 minutes with a fluoride toothpaste.
Brushing immediately after a meal can affect your enamel. If you consume any sugary or acidic foods and drinks - citrus fruits/juices, carbonated drinks (even the zero sugar options), then your tooth enamel is weakened. If you then immediately brush your teeth, this can cause erosion of your weakened enamel. It is advised that you wait at least 45mins before brushing your teeth after these foods and drinks.
You can introduce brushing after lunch, but just be weary about the timing of eating and brushing, especially if you've had anything acidic.