@Barnet-Sherman There is a status bar at the bottom of the Sound Forge window. It shows the bit depth, sample rate, channel number etc. of the currently selected file. Right click on the channels to change to a different setting.
If you wish to record a single-channel file from the get-go . Open 'File> New' and set 'Channels' to 1 .Though it may be easier to just record dual-mono and change it to single-channel in the status bar as @SP.stated.