The Nemean Lion
For the first of his labors, Hercules had to go out and kill a great lion that lived in Nemea (Greece). He strangled the lion with his hands and hit it with his club. Then he skinned it and after that Hercules always wore the lionskin around his shoulders like a scarf.
You might compare this story to the Jewish story from about the same time of Samson, who also kills a lion with his bare hands (and also is betrayed by his girlfriend). Both of these stories may be related to much earlier Sumerian stories about Enkidu (who is also tamed by a woman).
Twelve Labors of Hercules (Step into Reading, Step 3), by Marc Cerasini. Very easy, for beginning readers.
The Story of Hercules (Dover Children's Thrift Classics), by Robert Blaisdell (1997). Easy reading, very cheap.
Hercules, by Nancy Loewen (1999). More sophisticated, with a look at how the myth was passed on and what it meant to people, as well as the story itself.
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire. (Look under Heracles).
The Myths of Herakles in Ancient Greece, by Mark W. Padilla (1998). By a specialist, for serious readers.