The existing answer is accurate but very brief in explanation. Hidden Power is far more complex of a mechanic than can be explained by one table of IVs, so I'll try to detail how to get a strong Hidden Power Grass, and also why your particular Jolteon probably won't be able to get it.
As you may be aware, Hidden Power's type and base power are determined by IVs. (For those unaware, IVs are numbers ranging from 0 to 31 assigned to each stat, with each IV value being worth 1 stat point at level 100.) This is done through two mathematical formulas, explained here on Bulbapedia. If it looks difficult, here's a dumbed down version:
- Consider each of your Pokemon's IVs. If one ends in an odd number, replace it with 1. If one ends in a even number, replace it with 0.
- Plug the numbers into this formula (the same one as the one on Bulbapedia). Remember to multiply each stat's value by the number next to it:
((HP + 2Atk + 4Def + 8Spd + 16SAtk + 32 SDef) x 15) / 63
- Round the resulting number to the nearest whole number. It will be a number between 0 and 15. This number determines the Hidden Power type. On Bulbapedia, there is a table that will link the number with the type. For Hidden Power Grass, the result must be 10.
This is calculated similarly to type, but the formula differs slightly. Use the same calculation as above, but instead multiply the sum of the stats by 40 and add 30 to the final result, like so:
(((HP + 2Atk + 4Def + 8Spd + 16SAtk + 32SDef) x 40) / 63) + 30
Additionally, the numbers you plug into the formula are no longer based on the least significant bit (odd/ even) but on the second least significant bit. To find this for each stat, divide them each by 4. If the remainder of the calculation is 2 or 3, that stat's value (or bit in code) is 1. Otherwise, use 0. (This process means it's possible for every type to have 70 base power.)
The resulting number, rounded down, is Hidden Power's base power. As a consequence of 30 being added at the end, the power can range anywhere from 30 to 70.
Even if these numbers means nothing to you, and even if you don't know your Jolteon's IVs, you should take one key point from these processes, which is that there is a large amount of different type and power combinations available. With this in mind, and also the consideration that there is no legitimate way to change IVs, the probability that your Jolteon will be able to use Hidden Power Grass with 70 base power is low. For it to have Hidden Power Grass with 70 base power, it would need to have odd numbers in all its IV stats except HP and Special Attack, and every one of its IV values would have to have remainder of 2 or 3 when divided by 4.
If this still doesn't make sense to you, consider: the chance that your Jolteon will use Hidden Power Grass with a base power of 70 is extremely low. If it does not have the type and power combination you want, unfortunately there is no way to change that. If you don't know your Jolteon's IVs, you can't find its exact Hidden Power base power, but if you speak to one of the guys in the Veilstone City Game Corner (also where you can buy Hidden Power TMs), he'll tell you your Jolteon's Hidden Power type. Statistically, there's roughly an 8% chance that it will be Grass, and an even lower chance of it being 70 base power.