To set volume at the beginning of your track you can enter it in the Volume field in the Track Sheet.
To change volume anywhere in a track, use the Controller Editor. Select Volume from the Data Type list box in the control area of the Controller Editor. Select the Pencil tool and click where you want to enter a single volume change. You can also drag the mouse to enter a group of volume events. When you drag in volume changes, the density of the events is controlled by the step setting in the Step list box.