Plants prepare their food by the process called " Photosynthesis"
Photosynthesis is a chemical process by which glucose is produced in the plant leaves and this reaction also releases oxygen as by product. Both these elemetns are essential for all life to go on.
The process of photosynthesis takes place in plants leaves and in some algae.
The raw material required for photosynthesis is water and carbon-di-oxide.
Roots of the plants absorb water and salts from the soil and deliver it to plant leaves.
Undsersurface of the leaves have very tiny pores( holes) which abosorb carbon-di-oxide from the air.
The plant leaves have special cells called 'chloroplast' which produces a green pigment called 'chlorophyll'.
The name is derived from two Greek words : 'chloros' meaning green and 'phyllon' meaning leaf.
This chlorophyll absorbs light energy from the sun. Hence a leaf may be viewed as a solar energy collector full of photosynthetic cells. Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). Thus plants are the only site of food production.
When the plant leaf has all the raw 
material ( meaning water and carbon-di-oxide), a chemical process starts which combines six molecules of water and six molecules of carbon-di-oxide to produce glucose.
Since the energy for reaction is coming from light energy, this process is called Photosynthesis.
The Photosynthesis reaction can be simply put as follows.
6 molecules of water + 6 molecules of carbon-di-oxide ---------Light energy ---------> one molecule of glucose + 6 molecules of oxygen.
i.e.. 6H2O + 6CO2 ----------> C6H12O6+ 6O2
This glucose is utilized by plants as their food and oxygen is released in air.
Sugar which remains extra after the requirement of plant is stored in plants and it becomes the source of food for all the animal kingdom. Vegetarians get their food directly from plants and non-vegetarians get their food indirectly from plants as these plants are eaten by animals used as non-vegetarian food source.
This relation is well explained in natural food chain shown below.
Now we know from where all food comes, no matter whether one is Vegetarian or non-vagetarian ...
