If you already have a modern entry door, properly insulated, a storm door will not provide any significant energy savings. You do not need a storm door – except, say, if you plan to use it as a screen door in the summer, to keep insects out while breezes blow through.
If your entrance door is leaky, consider a new energy-efficient door; or consider weatherstripping it properly. You may not need a storm door.
Modern Glass storm doors Energy efficiency
Some modern storm doors are strong and reasonable at reducing air leakage, but most aren't. Many storm doors are "triple-track": they allow the screen and the glass to be shifted, to suit the season.
Some are mostly glass-doors, which may be useful to get natural light while the primary door is kept opened, or to catch breezes, but they will not reduce air leakage or heat transfer significantly. And they may damage the primary door: if a glass storm door gets too much direct sun during large periods, trapped heat may damage the main door.
Wood-Aluminum storm doors
“Traditional” storm doors with a wood core and an aluminum skin are a bit more advantageous from an energy efficiency standpoint. These doors may look good for many years, and can reduce air leakage when equipped with a good weatherstrip; and they can provide real protection.
Anyway, do not expect too much from these doors, from an energy-savings standpoint. Expect prices around $150 to $200 for this type of storm doors.
Installing a storm door is a typically DIY. Just follow the manufacturer’s instructions. Most storm doors are sold to fit standard 80 inches/2 meters high doors (32 inch/0.8m and 36-inch/91cm width).
Qualified storm doors
Currently, there aren't qualified storm doors. The National Fenestration Rating Council (NFRC) and the Energy Star programs for doors and windows are not including storm doors in their energy efficiency guidelines.