You will be told how much of each element you have. Check out this example:
E.g if they said: Find the formual of Iron Oxide when 44.8g or Iron react with 19.2g of Oxygen.
1. (get the moles) Moles = mass/RAM
So : Fe ------- O
Mass : 44.8 ------- 19.2
RAM : 56 ------- 16
Moles : 0.8 ------- 1.2
2. So youve got the moles. Next bit is to divide both numbers by the smallest number. E.g divide 0.8 and 1.2 by the smallest number (which is 0.8)
If you do that you get
Fe O
1 : 1.5
x2
2 : 3
(now you cant have 1.5, so double both numbers and you get the answer)
Fe2O3 ( NB- The O is oxygen, not a zero)