Buy the Pokemon Platinum guide. It explains it in there. I did and I have all of the pokemon, except for the pokemon received in distribution events. Thats around 482 pokemon in total, and I have one of each in the pokemon storage boxes. It took around 5 weeks, but it was worth it. Remeber that if you do not have all of the pokemon, you could trade with people over the internet or trade with friends who have platinum, pearl or diamond. Thats only if they actually have the game, or if they actually exist!
Happy Gaming
Happy Gaming