The .Value attribute of a checkbox will return True if it is checked and
False if it is not.  Therefore, you would use

If [checkboxname].Value = True Then
   'Code to do what you want to do in this case
Else
   'Code to do what you want do do in thsi case, if any
End if

If you do not need anything done if it is false, you can omit the Else part
of the If...End If construction

You need to replace

[checkboxname]

with the actual name that you have given to the checkbox.