convert analog to digital value, to send it to counter, see gallons accumulated in seconds

Hello everyone, good evening,
I have a program i need to visualizate  the total gallons 
that pass every second, the problem I have, 
and request your support, is to store that number of gallons in to a counter, and that value is updated every second. 
honestly I'm just starting in PLC LOGO!, 
can someone help me? 
I intend to make a gallon totalizer o flow totalizer ,
 with an analog input. and visualize them every second. 
I hope you can help me .... thank you