EV training: a primer.
Every pokemon can have a maximum of 510 Effort Values, or EVs. They can hold a maximum of 255 Evs per stat. You can invest EVs in HP, attack, defense, special attack, special defense, and speed. In every pokemon game past generation II, 4 Evs in a stat will give you one point in that stat, and since 255 does not divide evenly by 4, the most you can give a pokemon without wasting EVs is 252. This is the ideal amount to give a pokemon if you're gonna max out your pokemon's stats. Every pokemon has a set number of EVs it gives you when you faint it, which can be seen here. For instance, Gengar will give you 3 Special attack Evs when you defeat him.
Generally, you want to invest those EVs in stats that compliment the pokemon. If your Umbreon is going to be a wall, you shouldn't waste time putting EVs in his special attack.
One important note: Investing EVs in a stat will not necessarily make a pokemon good at that role. For instance, putting EVs in Infernape's defenses won't make him a good wall, even if you put full EVs in both defenses. His defenses will still only be 265 at the very most.(and that's with a beneficial nature!)
If you accidently give EVs in a stat you don't want, you can use the following berries to reduce your EVs and try again.
- #21 Pomeg Berry (HP)
- #22 Kelpsy Berry (Attack)
- #23 Qualot Berry (Defense)
- #24 Hondew Berry (Sp. Attack)
- #25 Grepa Berry (Sp. Defense)
- #26 Tamato Berry (Speed)
There are a few items listed on the EV guide link above that you can use to make things go a bit faster.
There's this question, but it ultimately links you to the EV guide on this site.