Okay, count each type of atom on each side of the equation.
There are
Pb - ? Ions
NO3 - ? Molecular ions
K - ? Ions
I - ? Ions
Count those on the left, and those on the right, and you get:
Left:
Pb - 1 ion
NO3 - 2 molecular ions
K - 1 ions
I - 1 ions
Right:
Pb - 1 ions
NO3 - 0 molecular ions
K - 0 ions
I - 2 ions
You see what problem there is? You forgot K and NO3. Since Pb 'stuck' to I, K will go with NO3.
Then, you need 2 I on the left, hence put a 2 as coefficient of KI and let's see what happens.
Count again. This time, you have one NO3 ion less on the right, and 1 K less on the right. So, put a 2 as coefficient in front of KNO3:
All is right now. Can you continue now? :)