P = Patient population
I = Intervention or topic of interest
C = Comparison that will be conducted
O = Outcome
There are certain that are "PICO" and others that are not. It's simply a way of phrasing your question. Your textbook should give you examples of how to phrase your question the right way -- to be sure you have identified the P, the I, the C, and the O.
Start by identifying your general topic of interest. Then go to the literature and see what types of studies on that topic in the past. Look for a topic for which there HAS been some research done over the past few years, but that still has some questions left to be answered.
So ... what types of nursing and nursing problems/issue interest you?