Jason, despite being a relatively inept hero himself, manages to get together a crew of Superfriends to help him on the journey. This group is called the Argonauts. Atalanta would have been one of them, except Jason was a little wary of having a woman on board and ended up refusing, tho reluctantly.
The Argonauts impregnate the entire island of Lemnos, where there are only women. This is unrelated to the rest of their journey.
After passing through various trials and bad stuff, they arrive in Colchis. Colchis is considered rather foreign to the Greeks, and so it is no surprise that its princess, Medea is the priestess of the witch-goddess Hecate. Falling in love with Jason, she betrays her father and leads Jason to the Fleece. (This also involves more tasks, but Jason uses Medea's help to get through them all.)