I’m new to speedrunning and want to speedrun the first boss category but I don’t know if I need a timer or not which is a problem since I don’t know how to set up a timer if I do.

You do not need to have a timer to submit a run. I retime runs either way. If you do want to have a timer on screen, most people use LiveSplit (https://livesplit.org/), although the autosplitter will only work for the pc version.