How does H2O and CO2 make glucose?

Most plants (and some bacteria and fungi) do this by a process called 'photosynthesis' (photo=light, synthesis=create).

Plants store the radiant energy from sunlight in high-energy glucose molecules through a series of chemical reactions that take place in tiny organelles in their cells called 'chloroplasts'. Incidentally, it is the green pigment called chlorophyll in these that traps the sunlight, making most plants green.

As glucose is composed of just carbon, hydrogen and oxygen which are all found in carbon dioxide and water, these chemical reactions can all be simplified to this:

6 H2O + 6 CO2 = C6 H12 O6 (glucose)