What constitutes a breakfast?

Before the actual eating, it is advised to clean the digestive track, in other words to get rid of the digestive rests from the day before. Drink a glass of lukewarm water as soon as you wake up. You can add a few drops of lemon juice if you would like. Take time to breathe deeply, do some stretching, observe your plants grow... before start eating.

On your table should be:

A cold or warm drink to hydrate your body: tea, coffee, herbal tea, water, milk, soy milk, fruit juice...
A protein source: it can be milk, a yoghurt, some cheese, custard... For vegans or assimilated it can be soy milk (calcium enriched), cereals, a soy yoghurt...
A fruit: either whole or juiced but be careful because commercial juices are often sugar added. The other problem with juices is that it is easy to drink too much and too quickly. Prefer fresh fruits which will also give you fibers.
Carbohydrates: those provide energy. During the digestion, carbohydrates slowly supply the starch which can help your body until the next meal. Carbohydrates are available from foods made with cereals, such as bread, rusks, rice cake, pancakes, buns, brioche, muffins, croissants...
Be careful those contains also a lot of fat.
For comparison, a croissant from a bakery weighs about 45g and contains 8 g of fats, whereas a slice of bread of the same weight contains only 4.5g of lipids.
In small quantities, you can eat some sugars (honey, jam, fruits...) and some fats (butter, margarine, nuts...).