I'm going to be brutally honest.
After looking at the natures you have posted for your Eeveelutions. Well, you're a long way off from grasping the basics of these specific pokémon - and I honestly doubt you'll get the best out of Jolteon (let alone the less straightforward Eeveelutions like Umbreon).
If you care about having the right nature: Get a Modest Mint. Jolteon is already fast so Timid is not necessary.
If you don't want to get a Mint and would rather breed: Jolteon is in the Field Egg-group, which is enormous. Just get a female Eeveelution and search your boxes for a Modest Pokémon. Chances are you'll find one that's in the Field Egg Group.
If you don't care about Natures at all, Mild is fine. At least it gives your Jolteon an offensive boost. The Defense drop from Mild is not that bad, as it's likely to die to most hits anyway since it's a paper towl defensively to begin with. Especially considering that the Ground-type is mostly physical. But Modest would be better since you shouldn't be using it's Attack-stat to begin with.
Additional tip: Catch a BUNCH of Indeedee or start breeding them if they've got Synchronize as their ability. It's a small investment, but leading off with either an Adamant or Modest Indeedee when you're looking for either a physical or special-based Pokémon gets you a long way. Yes, defensive pokémon might be better suited with a defensive nature, but each Pokémon will run at least one attacking move and you'll end up not using the other stat. Unless you've got a mixed set in which case stuff like Naughty, Lonely, Mild and Rash might become helpful, but that's another bridge to cross when we get there.
MOVES:
- Thunderbolt: Ghost94 has given the Denlist.
- Shadowball: Straight of the bat telling you that coverage is important and unfortunately, Jolteon doesn't get a lot.
- Hyper Voice: This is basically the last move with a decent power output that it gets in normal play. Unfortunately, it's a Normal-type move and often times your better off using one of the other two moves.
- Volt Switch: You do some chip damage, you send out something that can wall their next attack and they need to restrategize. Like if they're using a Choice Item, and you Volt Switch into something that's immune - they'll need to switch and you've gained momentum.
These are the moves that most people will tend to use or advise to use when thinking of Jolteon. But if you honestly don't see yourself commiting to online battling, then Hyper Voice and Volt Switch will rarely see usage.
Which leads me to this next section: Moves you personally could arguably be better off with for the 3rd and 4th slots.
- Yawn: If you're going to die and don't have that KO in range, use Yawn. It will either end up putting the opponent to sleep or if used online, it will cause a switch, which at least will cause some momentum change.
--- How to get Yawn on your Jolteon:
-------1. Catch a Male Wooper/Quagsire, it learns Yawn at level 21/23 based on the stage it's in (Wooper evolves in lvl20) so you're best off holding off the evolution for that 1 level.
-------2. Breed it with a female Eevee/lution. You'll now have an Eevee with Yawn.
-------3. Evolve that Eevee into Jolteon.
-------4. REMOVE a move from your Mild natured Jolteon at a Pokécenter.
-------5. Put the two Jolteon in the Nursery together and after a short while, you'll have both Jolteon knowing Yawn.
- Thunder Wave: It's easy to forget, but a paralysis can give you the win sometimes and could help a slower teammate get a win.
- Attract: Yup, that's how barren the moveset actually is.
If you do try out online battles:
- Weather Ball: ONLY ON DOUBLE TEAMS WHEN YOU ARE USING A WEATHERSETTER. Weatherball can offer additional 100bp Water, Fire, Ice or Rock-type coverage. Huge, since Jolteon's moveset is barren otherwise. But worthless in singles since Jolteon can't set weather itself and wasting a turn of weather to switch to Jolteon, well, chances are you're just wasting the turn of weather being up in the first place (which is why Hyper Voice is listed in the above set over WeatherBall). Also, IF you're using a RAIN-setter, you should change Thunderbolt to Thunder, since it loses the accuracy drawback in rain.
