The first name Eleven learns is her given birth name in "Stranger Things" is Jane.
Eleven was born Jane Ives, the daughter of Terry Ives. As an infant, she was taken from her mother by Dr. Martin Brenner and raised at Hawkins National Laboratory as a numbered test subject. The lab identified her primarily as Eleven, or 011, rather than by her birth name, so she grows up with little knowledge of her family or identity.
Her history is gradually uncovered through memories, documents, and contact with her mother. After Eleven closes the gate to the Upside Down in season 2, Dr. Sam Owens gives Jim Hopper a forged birth certificate. This allows Hopper to adopt her legally, and she becomes Jane Hopper while continuing to be known as Eleven or El.
Jane is often confused with her later legal surname. "Jane" is the birth first name; "Ives" is her birth surname, and "Hopper" becomes her legal surname after Hopper adopts her. Suzie and Angela are other characters, not alternate names for Eleven.