I think you are a little confused about EV training. It uses math, just warning you.
You gain EVs when you get Exp. You unconsciously EV train whenever you battle. So you have to start at LV 1. If you get to LV 50, you already have EVs.
Check this part of the site to find out which Pokemon will give you which EVs. http://pokemondb.net/ev/all
In White2 and Black2, you can EV train past LV 100 with Vitamins and stuff. But if you already have 252 EVs on a certain stat(the max per stat), you can't add more. People usually put 252 EVs in one stat, 252 in another, and then 4 in a last one. That leaves 2 EVs. Don't bother training for the last two, since you need four to get one stat point. (4 EVs=1 stat point) The last two would be wasted. A Pokemon can only have a total of 510 EVs, but people only use 508.
Use certain berrys to take away EVs, such as Pomeg Berry. The berry desciption tells you what EVs it lowers.
To EV train, you must battle Pokemon that give off the EV you want. For example, Stunfisk gives 2 HP EVs. To get 252 HP EVs, you could battle to battle 126 Stunfisks(126=half of 252). Make sure to write down how many EVs you already have! If you lose track, you're screwed.
Note: Stats are gained via EVs when you level up while having four EVs in that stat. Once you level up with four EVs in that stat, you must get four more EVs to get the stat again.
At first, it won't look like you are gaining much more stats than normal. But once the Pokémon is LV 100, you will see the differance.
