A lot depends on the particular I/O cards you're using, use the link JohnW provide to get the manual for the card(s) you plan on using. The manuals are pretty good, have setup information, example ladder and so forth.
After you've done that, give it a try, and come back with specific questions, someone will help you over the bumps.
The basic procedure isn't clearly identified in the manual. It isn't too tough, but you have to do a few steps:
1) Define the I/O card and the range and the channels being used in the I/O menu.
2) for inputs you can read the data directly from the I/O channel address. You should scale the data to get engineering units. The data range depends on the I/O card type and the way you set the configuration.
3) For outputs you need to convert the engineering units to data, and as with inputs the data range for 0-100% output signal depends on card and set up.
4) Within the program you can use compares for alarms or other logic, math operations on the data, PID control, whatever is required.