To balance a chemical equation, we want to make sure that the number of atoms of each element is equal on both sides of the equation. Here's the balanced form of each of the chemical equations you provided:
C6H12O6 + 6O2 -> 6H2O + 6CO2
Ba + 2H2O -> Ba(OH)2 + H2
2K + 2H2O -> 2KOH + H2
2Al(OH)3 -> Al2O3 + 3H2O