OK, so using Serebii's IV Calculator I've calculated that your Deoxys is in its Normal Forme, and its IVs should be around 31/15-16/31/31/18/25-26. If it was me, I would Soft Reset and try to get a better one. If you're beyond that point, though, keeping it is fine too.
Let's use the process of elimination to see how it should be used.
First of all, Deoxys-N is outclassed and shouldn't be used at all. So that's one forme down.
Deoxys-A requires max Attack, Special Attack, and Speed, which this Deoxys doesn't have. So that's another one down.
And last but not least, Deoxys-D is outclassed and also should be running a defensive nature.
So that means you're going to be running Deoxys-Speed!
TL;DR Soft reset it if you can, if not, use Deoxys-Speed with the following set:
Deoxys-Speed @ Focus Sash
EVs: 252 HP / 4 SpD / 252 Spe
- Stealth Rock
- Knock Off
- Spikes / Magic Coat