First, you breed it with Ditto until you get a female turtwig (the female determines the pokemon type, the male detemines the moves)
Then breed the female turtwig with a male bulbasaur/ivysaur/venesuar, Eggsecutor, Sunkurn/Sunflora or Shroomish/Breloom that knows seed bomb.
It also can learn it through breeding with the following types of pokemon evolution lines who learn it through breeding: hoppip, roselia, cacnea, cherubi, snover, maractus, feroseed, and a turtwig that has already learned it from breeding.
There also are a lot of types it can breed with who learn it from a gIV tutor, which you can see with the following link.
http://www.serebii.net/pokedex-bw/egg/389.shtml#seedbomb
