Take your mouse to the map image and start scrolling.
In the Project Toolbar (at the top of the screen), go to the magnifying glass displaying the plus sign.
Then draw a box around the area in the map you want to zoom in on.
Go to the Scale window below the map image and choose a scale or enter one manually.
This scale is the scale as it will be shown in the Print Composer.
How the scale appears on your screen will obviously depend on the size of the screen and the Qgis window.