Storm Spirit is a highly mobile ganker and semi-carry. Although his abilities deal magic damage, Storm actually scales well into the late game thanks to his incredible mobility and combo potential.
Skills
Skill Static Remnant first but max it last.
Skill Electric Vortex last but max it first.
Skill Overload second and max it second. Remember to try to right click the enemy after every spell to get the most out of it.
Skill your ultimate, Ball Lightning at level 6, your all-important mobility skill which allows you to gank and escape.
Items
Starting Items: A set of Tangos, a Healing Salve, Mantle of Intelligence, and 2x Iron Branches.
Early Game: Bootle, Power Treads, Null Talisman, Magic Wand.
Core Options: Orchid Malevolence (offensive, competitive build) or Bloodstone (safer, old school build).
Luxury Extensions: Black King Bar, Orchid Malevolence, Bloodstone, Scythe of Vyse, Shiva's Guard, Eye of Skadi.
Situational Items: Linken's Sphere, Dagon, Boots of Travel, Arcane Boots.