The mass of the products = the mass of the reactants.Always know the Left hand side and Right hand side must have the same amount of atoms for each element.
e.g; SnO
2 + H
2 Sn + H
2O - From this, you can see that on the Right hand side there is only 1 atom of Oxygen, while on the Left hand side, there are 2. Put a co-efficient of 2 in front of H
2O.
SnO
2 + H
2 Sn + 2H
2O
Although we have balanced the Oxygen, we have unbalanced the Hydrogen and must balance that next.
All there is left to do is balance the Hydrogen, to do this, simply place a co-efficient of 2 on the Left hand side in front of Hydrogen.
SnO
2 + 2H
2 Sn + 2H
2O
Our final equation consists of:
1 Tin
2 Oxygen
4 Hydrogen
On either side
Hopefully this can help you wrap your head around the whole idea of balancing Chemical Equations.
If there are any specifics that you need help with, please post them.