Staryu and Starmie are incapable of learning the move Confusion by any means other than cheating. If you mean Confuse Ray, then Starmie learns it at level 28 in Generations V and IV, level 33 in Generation III, and level 37 in Generation II. Staryu never learns it except in the Trading Card Game.
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LEVEL 7,8,9
Butterfree learns confusion at level 12.
You can evolve Staryu at any level however it learns it's last move at level 52. The best time to evolve Staryu is to decide what moves you want it to have and see if it can learn any of them from leveling. Once the Staryu has the exact moves you want it to have as a Starmie, then is a good time to evolve it.
lv 16
In Red and Blue, it learns Confusion at Level 12, two Levels after it evolves from Metapod. In every other game, it learns Confusion at Level 10, as soon as it evolves from Metapod.