I believe Serebii.net can do a way better job at that for
EVs/
IVs.
As for natures, it's a lot more simple.
There are 25 natures in total, 5 of which are the neutral ones (Bashful, Docile, Hardy, Quirky and Serious).
These 5 have no extra effects on the stats you are gonna get on a Pokemon.
The other 20 natures however, will affect one stat positively, and another stat negatively.
That is to say, when a Pokemon with one of the 20 natures... Say Adamant, levels up. By it's nature, it's Attack stat will grow faster than normal, while it's Special Attack stat will grow slower than normal (normal as in terms of the 5 neutral natures).
The list is as follows:
[spoiler=The 20 natures...]Lonely: +Att, -Def
Adamant: +Att, -Sp. Att
Naughty: +Att, -Sp. Def
Brave: +Att, -Spd
Bold: +Def, -Att
Impish: +Def, -Sp. Att
Lax: +Def, -Sp. Def
Relaxed: +Def, -Spd
Modest: +Sp. Att, -Att
Mild: +Sp. Att, -Def
Rash: +Sp. Att, -Sp. Def
Quiet: +Sp. Att, -Spd
Calm: +Sp. Def, -Att
Gentle: +Sp. Def, -Def
Careful: +Sp. Def, -Sp. Att
Sassy: +Sp. Def, -Spd
Timid: +Spd, -Att
Hasty: +Spd, -Def
Jolly: +Spd, -Sp. Att
Naive: +Spd, -Sp. Def[/spoiler]
Natures are important if you want specific kinds of Pokemon.
For example, you know Machamp has a great Attack stat, and you want to further improve on that Attack stat, so Adamant (+Att, -Sp. Att) will be a great choice since Machamp usually won't be using Special moves.