Complex sentence: Born in Alonsotegi, Biscay, Goikoetxea began his playing career as a youth with "Arbuyo", before joining Athletic Bilbao in 1974--before establishing himself in the senior team, he played for the reserve side.
Split into simpler sentences: Born in Alonsotegi, Biscay, Goikoetxea began his playing career as a youth with "Arbuyo", before joining Athletic Bilbao in 1974. Before establishing himself in the senior team, he played for the reserve side.