Serperior is a new grass type pokemon in Pokemon Black and White. Serperior evolves from a Servine at level 36, which evolves from a Snivy at level 17.
Serperior is only attainable from evolving the starter pokemon Snivy into Servine, and then into Serperior, none of which can not be caught in the wild.
If you want all three starter pokemon and their evolutions, you’ll need to get to the first pokemon center where you can trade pokemon. Trade your starter, then restart your game. Do this process twice picking different pokemon. On your third restart, have all of your starter pokemon traded back and you’ll have all three starter pokemon.
