I really recommend doing the Introduction to ThingWorx training course as this covers how to use both Streams and ValueStreams.
All you need to do when you create a ValueStream is to give it a name. You then link the value stream to the ThingTemplate for the Things that are to log data in the valuestream. In the property definition of the ThingTemplate (or the ThingShapes that define the properties of that ThingTemplate) set the properties that you want to be stored in the ValueStream to be "Logged".
Hope this helps