Draw out the structure for the substances on the left hand side. (for structures see wikipedia)
Count the different bonds, and remember to scale up by the coefficient.
Repeat for right hand side.
eg.
2H2 + O2 --> 2H2O
so,
LEFT (bonds broken)
H-H -> 2
H-O -> 0
O=O -> 1
RIGHT (bonds formed)
H-H -> 0
H-O -> 4
O=O -> o