Storm is a Marvel Comics character associated with the X-Men and other groups. She has appeared in comics, animated television shows and live-action movies, where she was played by Halle Berry. Since her first appearance in 1975, Storm has gone through many makeovers and overhauls. This gives you quite a bit of leeway when creating a Storm costume.
The easiest way to dress up like Storm is to simply rent or purchase a costume from a store. Many costume stores have X-Men costumes, though you may have difficulty finding anything but the movie versions in adult sizes. You can also search online costume sites which sell Storm costumes. These sites will give you more variety to choose from but you will likely not have the less expensive option of renting the costume.
If you have a talent for sewing, you can make your own Storm costume fairly inexpensively. She has had many costumes throughout the history of the comic book but two of the more well-known ones are her head-to-toe silver outfit with cape, held in place with two X symbols at the shoulders, and her skimpier black costume which can be recreated with a black sports bra with yellow collar sewn on, tight black trousers, yellow elbow-length gloves, yellow knee-high boots and yellow kneepads.
One thing that Storm has in almost every incarnation is brilliant white hair. If your hair is fair enough, you can use white hairspray from a costume shop to temporarily add the colour. Otherwise you will need to use a white wig. In the comics and movies Storm went through many hairstyles from incredibly long to short and choppy to mohawk, so the style is not as important as the colour.
When Storm uses her powers, her eyes traditionally go completely white. This can be recreated with speciality white contact lenses, available through many contact lens sites. The more reputable sites require you to have your eye doctor send them your measurements so that lenses can be made to fit you correctly. This greatly reduces the risk of eye injury. When you buy the contacts, make sure that you will still be able to see. Some are so opaque that they will make you effectively blind.