Designing a PI controller for my power system
I suppose you don't have the P(s) transfer function. It can be an academic problem or an exercise over a real plant which an unknown model, but at least you should know WHAT INPUT makes P(s) to have that output. Is it an impulse, a step, a sine?

Lacking mathematical knowledge, you can follow some empirical rules, well spread in the industrial field, google for "ziegler nichols tuning method" or begin here.