you should start EV Training as soon as you get the Pokemon, as long as it isnt traded. To add EVs you can use many things. Vitamins are good at the start. Buy 10 of them and use them immediately. That's 100 EVs down. To get the remaining 155 go find wild pokemon that give off the certain EV you want. Items like the Macho Brace and Power Items help, and so does PKRS.
Macho Brace doubles the EVs.
Power Items add 4 EVs with every battle.
PKRS also doubles the amount of EVs.
Here is an example: Bibarel gives off 2 Attack EVs. If your Pokemon has PKRS and a Macho Brace it will gain 4 times as much EVs.
2 (Bibarel) x 2(PKRS) x 2(Macho Brace) = 8 Attack EVs in total. Just record these and stop when you reach 155