Go to
http://ojp.nationalrail.co.uk/en/pj/asfc
Fill in the details for you journey, from, to, and the dates and times.
Scroll down and select the number of adults and number of children.]
Click on 'search'
You'll see details of the available journeys.
Click on 'check fares'.
It will show you a table of the single and return fares available, and the types of ticket.
Scroll down to the 'return fares' section, and look at the off peak fares. This is the return fare you'll have to pay.
If you scroll up a bit and click on 'see all details' it will show you the changes required for each journey.
So now you know how to do it I don't expect you to ask any basic train fare questions again.