The shortest mate is called "Fool's mate" and requires the cooperation (or lack of skill) by the opponent. It requires 5 moves (3+2) for White to mate Black, but only 4 moves (2+2) for Black to mate White.
Fool's mate by White
1. E3 f6
2. * g5
3. Qh5 mate
(*any move except Q-h5 or one that blocks the diagonal for the queen)
Fool's mate by Black
1. f3 e6
2. g4 Qh4 mate
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Two Moves, also known as the fool's mate (see link). Unless you mean, how can you always guarantee to win (no one can answer that).