Me too ^^
I haven't gotten around to EV training myself in white but i know a few spots:
Sp. Attack- Celestial Tower (the candle pokemon)
Attack- Route (the dog and patrat)
Speed- Route 3 [surf] (basculin)
HP- Bridge (the duck)
Sp.Def Route [surf] (frillish)
I dont know a good place for defense though, you can try wellspring cave for roggenrola or the desert cave for yamask, but that's only 50% encounter rate.
Best time? Right when you catch or hatch your pokemon. They must gain the evs before any other kind of battling (besides wifi or frontier battles) or exp training.
Do you need to know the best places to train? I got many spots to ev train.
Four zubats is four evs.
A zubat is one speed ev.
A golbat is two speed evs.
A crobat is three speed evs.
If you get four speed evs, your speed stat will go up by one point. So, battling four zubats is the same as battling one crobat and a zubat. A single individual pokemon can can only gain 252 ev points for one stat max, so a pokemon can battle 252 zubats, and its speed will be up by 63 points! That can make a big different in battles.
Battling a machop gives your poke 1 attack ev. After four of them, your attack stat goes up by one. A machoke gives two attack evs, and machamp is three evs.
A single pokemon can only acquire 510 evs total, so that means it can have 252 in one stat, 252 in another, and 6 extra in a third stat, which four become a stat point, leaving two evs leftover which have no effect. This is called ev spread. It doesn't have to be 252/252/6 all the time, it can be any amount you want, but it can't go more than 510 evs. Some items add additional evs gained by a pokemon when they earn them, and pokerus doubles evs altogether. Still can't EVER exceed 510 total or 252 for a single stat.
Get it? It's a lot simpler than the way i learned it, which is the hard way -__-