There are three major factors to consider when choosing ski poles: the fit, the strap, and the basket.
To find a pole with the right fit, stand in your ski boots and hold your pole at your side. Your pole is the perfect length if your arm sits comfortably at a 90-degree angle. You need a longer pole if the angle is smaller than 90 degrees.
The basket -- the piece of plastic or rubber that acts as a stopper at the base of your pole -- can come in a handful of shapes and sizes.
The basket shape best suited for you depends on the type of skiing you will be doing. As a general rule of thumb, use a larger basket with more surface area for powder conditions and a smaller one on hard-packed snow.