The task group page handles ordering ships around, first I'll assume your constructed freighter is in your default shipyard task group, select that task group in the drop down at the top, you should see your freighter and any other ships you've built there.  Select the freighter in the list to the right, then click the detatch button, your freighter is now in its own group named after itself.
Next select earth on the bottom left destination window, right of that is a list of valid orders, any ship with cargo holds will get options to load or unload; infrastructure, installations, minerals, etc.
Select 'load infrastructure', notice the window right of this now has an order listed 'load infrastructure at earth' , next select mars and then 'unload infrastructure' , finally select earth again and give an order to refuel.  You can select the checkbox for 'repeat orders ' to make the task group continue repeating this until it finally fails to load infrastructure when earth runs out.