You have to reset your EVs. To do this, you need to use EV reducing berries (Pomeg, Kelpsy, Qualot, Hondew, Grepa, Tamata).
After you've used those berries (prepare 11 for each stat) then you'll be able to use the IV calculator for your Pokemon. You can get those berries from the Florist's at Join Avenue and the Breeder on Route 5.
If you alternatively want to know the vague IVs of your Pokemon, go into the battle subway, and the Ace Trainer standing near the middle pillar will give you a rough idea of your Pokemon's IVs.