If you're looking for the first species that could walk upright *occasionally* for short periods, that would be Australopithecus afarensis (famous for "Lucy") who lived around 3.9 to 2.9 million years ago. They had a mix of ape-like and human-like features, and could walk upright but also climbed trees.
If you're looking for the first species that walked upright as their *primary* mode of locomotion, then the answer is likely Homo habilis who lived around 2.4 to 1.6 million years ago. They had more human-like proportions, longer legs, and a more developed pelvis suited for bipedalism.
However, it's important to remember that evolution is a gradual process and there were likely other species that developed this ability to a lesser degree. There's still much to learn about human evolution, and new discoveries could change our understanding of which species was the first to walk upright in a more definitive way.
