i dont have a rod and i picked tepig first how do i get a water type pokemon in black version
The First Water Type you can obtain is Tympole, that is obtained outside of the Pinwheel Forest. Simple as that.
You don't get the Super Rod (which is the only fishng pole in the game) until you beat the story, and you won't get HM03 (Surf) until you get to Twist Mountain.
You can get your first Water-Type Pokemon WITHOUT Surf and the Super Rod in Icirrus City. Just walk in the puddles. Check the second link for info on the Pokemon you can find.
For more info: Bulbapedia/ Twist Mountain
For info on Icirrus City : Bulbapedia/ Icirrus City
In pinwheel forest you have a small chance to find a PANPOUR panpour is a water pokemon http://pokemondb.net/pokedex/panpour , there you go.
on the bridge to driefveil city there are a few black circles moving around if you go onto them you might be able to find sawnna (sawnna is a water and flying type pokemon) or maybe you will find ducklett instead (ducklett is a water and flying type pokemon to) but most of the time you will just find feathers