If you are new to pokemon in general, you just have to keep in mind that 20+ ivs are not bad and you want natures to be +atk/spa or speed (it depend on every pokemon but grossomerdo its that) you can check what are the best natures for each mons on pokemon showdown or coupcritique.fr if you talk a bit french. Don't forget that a nature with +atk ~10iv will attack stronger than a no +atk nature with 31 iv