Electro was formerly known as Maxwell Dillon who gained electric powers when he was struck by lightning while working on a power line, he then was electrocuted by the cables. He survived and found out about his new electric powers.
When he first found his powers he wasn't very strong and stole electric supplies from Stark Industries to help raise his electricity capacity in his body so he could use more dangerous and powerful electric powers. He became an enemy of Spider-man after he broke into the Daily Bugle and stole Jonah Jameson's safe. Jameson accused Spider-man of having an alternate personality so Spider-man went after Electro.