Oh, it is very simple.
First of all you can do different kinds of charts in MS Excel and then insert it in word document.
But if you want to use Word only go to Insert - Picture - Chart. To change diagram type Rt click to chart area and choose chart type - Line.
But this option in Word is also related with Excel and it could be difficult to explain you how to execute it if you haven't had experience with Excel.
However if you want to here more detailed answer don't hesitate to ask.