The official spelling of the team is "N Team," as established in Season 2's "The Big Game."
It is unclear when the N Team was first formed. Based on Jeffrey Scott's script-writing order, Mother Brain first refers to the heroes as the N Team in "Videolympics." It is unknown whether the heroes referred to themselves as such beforehand, or they decided to officially adopt Mother Brain's name for them afterward.
The hierarchy of the N Team is never made clear in the series.
Kevin, being Captain N, is often assumed to be the leader. However, Lana is the ruler of Videoland and should, therefore, outrank Kevin. Lana is the one that comes up with the plan of action in "Mega Trouble for Megaland" and states it as if expecting it to be carried out. In "The Lost City of Kongoland," Kevin looks to Lana for approval to try out some weapons. Therefore, it can be deduced that Kevin is the field commander, but Lana is the overall leader of the N Team.
The ranks of everyone below that are not as obvious, however.
In "Kevin in Videoland," Simon refers to himself as "the highest-ranking officer in Her Highness' court" and assumes command of the mission to rescue Lana. However, this could merely be Simon overplaying his role.
It can be deduced that Simon is just below Kevin in the N Team's hierarchy, followed by Mega Man and Kid Icarus (in either order), if only because Mega Man and Kid Icarus are left behind to guard the Palace by Lana and Kevin.
Duke would seem to be next, and Gameboy (the late-comer) would be last. N-Team.