I would not prune any. Your plant looks like maybe it is one of the hybrids. The skinny round stems are probably from when the plant was either too dark or too dry for an extended period. New ones will arise from the bottom. They tend to be quite spiny, but will mature into what the bigger stems look like. I bet the wider leaves bloom within a year. Keep it warm and damp in the summer, a bit drier in the winter. Does it have any spines at all in the notches?