Hidden Powers are based on IVs. In the previous Generations, it was impossible to change this and you were stuck with the same type. Now, with the introduction of Hyper Training, you can manipulate them to an extent so as to get Fire. This is still unfortunately very hard to do for this simple reason:
where a, b, c, d, e, f (the "type bits") are the least significant bit of their respective IV's. If a number is odd, its least significant bit is 1, and it is 0 otherwise.
a depends on the HP IV.
b and c depend on the Attack and Defense IV's respectively.
d depends on the Speed IV.
e and f depend on the Special Attack and Special Defense IV's respectively.
For [Fire], the resulting value must be 8, so by a bit of reverse calculation,
(8 x 63)/15 = a+2b+4c8d+16e+32f nearly equal to 33 (Actually, it's 33.6, but you must round it down, so it's 33, not 34)
Here I realized it was still tedious work to calculate the possibilities, so here they are, pulled from this page:
Note that you don't have to have the exact values, i.e. wherever it's 31, an odd IV will do and where it's 30, an even one will suffice. This unfortunately means that you need to know the exact IVs of your Venusaur beforehand and how to get them to exactly the next value or to leave it alone.
And that's just the type.
Power calculation are still harder to manipulate, which you can see in the first source provided above.
All in all, it's still extremely hard to get exactly what type and power of Hidden Power you want, so you're better off using another move set, or trading for one with Fire.