a) If I meet him, I want to tell him how much he has influenced me.
When the conditional clause is at the beginning of the sentence, the realization of the objectives of the main clause seems to be
left to chance.
Whether I want to tell him how much he has influenced me depends upon whether I meet him.
b) I want to tell him how much he has influenced me if I meet him.
When the main clause comes before the conditional clause,
its objectives are to be carried out once the conditions are met.
I have made up my mind to tell him how he has influenced me and wait for the chance.