Born to the mortal woman Alcmene, Heracles was the son of Zeus, who had appeared to her in the form of her husband Amphitryon. Nine months later, Alcmene gave birth to twin boys, Iphicles and Heracles -- one the son of her mortal husband, the other a demigod. Heracles soon displayed his enormous strength, killing two serpents that jealous Hera had sent to slay him, though still in his cradle.1
As he grew to manhood, Heracles' great strength, and corresponding temper (one version of his story has him killing his music teacher Linus), caused his mortal father Amphitryon to send him into a contrived exile, guarding Amphitryon's flocks on Mount Cithaeron.
While doing so, Heracles hunted and killed the Lion of Cithaeron, a ravenous beast that had been decimating the flocks of not only Amphitryon, but also of the king of Thespiae. Having slain the lion, the young hero fashioned a cloak for himself of the beast's skin, and made a helmet from its head. All depictions of Heracles show him wearing this garment, along with his characteristic weapon, the club.2
1. Pindar: Nemean Odes. I.38.ff; Diodorus Siculus: iv.10; Apollodorus: ii. 4.8
2. Some versions of the myth have the Lion being killed, instead, by Alcathous, son of Pelops (Pausanias, Description of Greece i. 41. § 4). Likewise, some versions cite the invulnerable Nemean Lion as the source of Heracles' lionskin armour.