rookie

aloittelija, noviisi An inexperienced recruit, especially in the police or armed forces.
A novice.
An athlete either new to the sport or to a team or in his first year of professional competition, especially said of baseball, basketball, hockey and American football players.
puhekieltä A type of firecracker, used by farmers to scare rooks.
non-professional; amateur

The game was going well until I made that rookie mistake.