Pokémon X and Y Walkthrough, Pokémon Move Sets: Jolteon
(Please note that the recommendations below are largely made for in-game play. A Jolteon in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If your Jolteon has a different moveset than what's recommended below, by all means, list it in the comments.)
Good ol' Jolteon. Dependable since the first generation, Jolteon was long considered one of the fastest pokémon around. Others have since usurped its place at the top, but Jolteon remains remarkably swift and horribly powerful, so long as it's up against the right opponents.
One of the classic stone evolvers, Jolteon starts off as an Eevee. In order to get a Jolteon you need only give your Eevee a Thunderstone.
Electric, Flying, Steel.
Jolteon is one of the original sweepers from the first generation, and its stats still reflect that role. It is a Speed king, boasting base stats in sheer agility that will put the majority of pokémon to shame. It's quite difficult to go first when a Jolteon is on the field. Special Attack comes next, and is quite formidable, with Special Defense clocking in third at a surprisingly high (though still not that amazing) 95. Everything else is below average, which, given that ground-type moves are largely physical in nature, can make for some troublesome encounters if your Jolteon doesn't manage a OHKO. Ground-types are Jolteon's bane in more ways than one.
Volt Absorb: If this pokémon is hit by an electric-type move it will regain HP rather than taking damage. Volt Absorb is fun for irritating electric-type users who can't anticipate Jolteon switching in, but it's otherwise just a so-so ability.
Quick Feet: If the pokémon is hit by a status ailment, its Speed is boosted by half. Quick Feet would be a heck of a lot more useful if Jolteon could be hit by paralysis, as its Speed Boost ignores paralysis' usual slowdown effect, but, alas. Not that amazing overall unless you can predict poisoning or burning. Quick Feet is a hidden ability.
Your typical Jolteon will try to blast opponents into oblivion. To do this it will use Thunderbolt, perhaps the most famous of electric-type moves that it can learn (though, oddly, not naturally). Bolster Thunderbolt with other high-damage special attacks - Shadow Ball and Hyper Beam, primarily - and you'll be capable of dishing out a heck of a lot of damage to any pokémon you face. Toss in Thunder Wave for those irritating occasions when you know your Jolteon's repertoire won't be enough to wipe out a pokémon in one hit.
The alternative here doesn't move away from Jolteon's sweeping role, but it does change your moveset a bit. Swap out Thunderbolt for the more powerful, but usually less-accurate, Thunder. Give Jolteon Rain Dance to make Thunder perfectly accurate and lead with a rainfall whenever you bring Jolteon out. With some exceptions, this move is usually enough to wipe out enemies, super effective or otherwise. Thunder Wave is again useful for this moveset if you need some time to stall.
(If you don't want to take up a space in Jolteon's repertoire with Rain Dance, or if you don't want to risk it being wiped out on the first turn, consider recruiting a pokémon that knows Drizzle. Switch out to Jolteon when it goes down and Thunder will be ready to go on the first turn.)
EV / Super Training
As a sweeper, Jolteon has two EV training goals: a high Special Attack and a high Speed. Boost these two stats and Jolteon should do just fine. You should only consider changing priorities if you decide to use the Thunder and Rain Dance combo, in which case boosting either Defence or Special Defence is preferable.
Catching a Jolteon
You can find Eevee, Jolteon's predecessor, irregularly on Route 10 of Pokémon X and Y. Once you have an Eevee, hunt down a Thunderstone - these can be bought at one of the stores in Lumiose City - and use it on the Eevee to get a Jolteon.