Here I will guide you on what ev's are and how to ev train your pokemon. SO here we go. EV's stand for effort values and they make a huge difference in battle as they can be the deciding factor in a game. Now, you can put 510 max evs into a pokemon. 255 maximum into a stat, but it is recommended 252, since it is divisible by 4. Here is a list of the power items that will boost your evs by 4.

Power Weight-HP
Power Bracer-Attack
Power Belt-Defense
Power Lens-Sp.Attack
Power Band-Sp.Defense
Power Anklet-Speed

Now, the amount and type of evs given depend on the pokemon. The ev type will be explained later. For now, I will talk about amounts. When there is a NFE poke like starly, it gives one speed ev. Staravia gets 2. Staraptor is 3 attack. So the ev goes up one for each evolution.

Pokerus is a virus that actually helps your pokemon during ev training. It doubles the evs you receive. Back to the starly, Starly gives one speed ev + the power anklet= 5 evs then x2 because of pokerus= 10 evs for one starly. AMAZING. SO basically its (1,2,or 3 + 4) x 2= Amount of evs given.

HP

There are a few common pokes that give out HP evs. BIDOOF are very common. SHELLOS are also an oppurtunity. There is a girl trainer named Cowboy Shelly south of Solaceon who battles you with 5 or 6 bidoofs around Lv15.

Attack

There are a few options for attack evs. The SHINX family in the early part of the game near Sandgem Town are a good option. GYARADOS are another option for later in the game. Lucky if you find a STARAPTOR. MACHOP and MACHOKE are also great as well.

Defense

Defense is easy. Any GEODUDE, GRAVELER, ONIX, or STEELIX would do. Iron Island is great to goto.

Special Attack

GHASTLY and HAUNTER are the best to goto. You find them in the Lost Tower and Old Chateau.

Special Defense

Again very simple. Surf for TENTACOOL and TENTACRUEL. MANTYKE are also good options as well.

Speed

Well STARLY and SATRAVIA. Also ZUBAT and GOLBAT. But your best option is a fisherman west of Eterna City, the third one from the left. He has 6 MAGIKARP.

Well this is it. It's a little short, maybe long depending on how you look at it. Hope this helps!