Well for starters, Your gonna want your good natures, if you have no clue what natures are, go to Serebii. You are also going to want to fight Pokemon that give you certain points to what you are training. Like for example, a Magikarp would give you 1 speed point if you were to KO one. Now these points add up per level, so you are going to need to fight much lower level Pokemon than you. About every 4 or 5 EV points is 1 stat point. Check Serebii for certain Pokemon. They will show the EV points they give and how much. The stat you are trying to level should be the EV point you go for. Remember you only get 510 ev points so spend the all. Some others might have more to add, but that should get you started. (Side note, Rare Candy give 0 EV points so just getting a good nature+Rare Candy will not equal good stats. Another thing is that EV points stop around level 50.) I would also advise IV training if you are going to EV train. This will make max stats possible.
