Fins

THE FINS:    

No other piece of equipment has a greater affect on your diving performance than your fins.  Not only do have to be very powerful to get you back up again when your suit has lost its buoyancy but they also need to be light and efficient so you don't have to burn too much oxygen driving them.  As always fit is supremely important so try on different brands of footpockets as they will all have slightly different shapes.  Never buy open-heel, strappy fins.  They are less efficient and the buckles create a huge entanglement risk.

MATERIALS:  There are basically three types of blade; plastics, fibreglass and carbon-fibre.  Plastics are the most basic and cheapest but simply do not have the snap or performance of composite blades.  Two-part fins are a good choice to start with as it is possible to change the blades out later on.
When looking at composite blades, get ones that have a tapered blade as this creates the most efficient curve.

STIFFNESS:  The stiffness of the blade plays a large role in the performance of the fin and therefore the diver.  When talking about plastics, green blades are softer than black ones.  The heavier you are the stiffer the blade you will need.  It does not matter how strong your legs are.  Softer blades are generally easier to use while surface swimming while stiffer blades are generally better for deeper diving.
A very arbitrary guideline for choosing green or black blades is people under 85kg should use greens and people over 85kg should use blacks

Other factors that influence fin performance are the angle of the toe.  Most fins are either flat (better for finning underwater) or around 30 degrees which is better for surface swimming.  Full length rails are also good as they prevent water slipping over the side of the fin.