Servine is a new grass type pokemon in Pokemon Black and White. Servine evolves from a Snivy at level 17, and into Serperior at level 36.
Servine is only attainable from evolving the starter pokemon Snivy, 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.