well species is a type of Pokemon, like dragonite, blastoise, gyarados and suicune are all species of water type Pokemon. Although since you have white and you've just started out, you probably can't get any of those because they are from previous games before white. to get these you need to have beaten the game and received an upgraded pokedex that has all the Pokemon up to now. so in Pokemon white, the best water Pokemon is probably samurott, the evolution of one of the starter Pokemon, oshawatt. I have a Japanese samurott i can give you, level 100, if you want :)
oh yeah, and natures are one of the characteristics of you Pokemon, they affect you Pokemon stats by increasing one, and decreasing another. you can find you Pokemon nature by checking their summary. for example, adamant nature increases attack, but lowers special attack.
there, hope that helps, and if you want the Samurott, just drop me a line :)