To open the Taper tool, double-click on "Taper" in the Toolbox.
Enter percentage values in the Taper Factor control to taper any
selected points along the current Active Axis.
Consider the following object:

Here is the object after setting a Taper Factor of 0.33, with
Displace Y enabled and the X Axis Active:

Note that the points in the object have been progressively scaled
along the X Axis, and that only their Y coordinates have been scaled. If you enable
Displace Z, youll see the following:

Now, both Z and Y coordinates have been scaled up to 33 percent
along the X axis. Note that the scaling increases based on each points distance from
the object Origin. If you set the Origin to "X Min", youll see the following
(now flat-viewed along the Z Axis).

To the right of Taper Factor is a ControlChannel, allowing you to alter how the
points are scaled, based on their distance from the Taper Origin.
Note the following change to the ControlChannel,
and its effects on the resulting tapering:


To interactively taper selected points, LMB Drag in any Camera View while Taper is the Current Tool. As you drag the mouse, any
selected points will be tapered along the Active
Axis.