Its been asked so many times in the past, a simple googling would bring you up a decently written guide.
Effort values, simply known as EVs are a set of points that a pokemon would gain when it defeats another pokemon. For every 4 EV in a stat, a pokemon will gain a bonus +1 in that particular stat every it levels up. Different pokemon gives different types of EV. For example a wild starly gives 1 EV in speed while a wild geodude will net you 1 EV in defense. The bonus stat gain from EVs are accumulated on the way to lvl 100. So at level 100 a pokemon with 252 EV in attack will have 63 extra attack stat than a pokemon without EVs. A wild or hatched pokemon will start with 0 EVs in any stats.
In depth guide:
https://www.serebii.net/pokedex-rs/ev.shtml
IVs are a set of predetermined stats randomly generated when a pokemon is captured or hatched. They are normally hidden so you can't see them without an IV calculator. Basically, a pokemon can have from 0 to 31 IV in all six stats (HP,Atk,Def,Spatk,spdef,speed). A pokemon with 31 IV in attack will have 31 more extra attack stat than a pokemon with 0 IV.
In depth guide:
https://www.serebii.net/games/ivs.shtml
Natures are less complicated. They are divided into 3 categories: Helpful, Hindering and neutral.
Helpful nature increases a particular stat of a pokemon by 10%. Hindering nature on the other hand does the exact opposite. Neutral nature as you guess doesn't increase or decrease.
Adamant nature is a Helpful nature for physically based pokemon as it increases attack stat by 10% while decreasing special attack stat by 10%.
Modest nature is a Helpful nature for specially based pokemon as it increases special attack stat by 10% while decreasing attack stat by 10%.
Bashful nature is a neutral nature as it causes no change in stat distribution however.
For a complete list of natures and their effect see this table:
https://www.gamefaqs.com/portable/ds/file/925601/48715
Hope this helps.
