Alright, I think I should have made it more clear (re-read it and it seemed a little -lax- as far as relevant information)
Part 1- IV's and Hidden Power
As stated, you pokemon's IV's are set in stone the moment it comes to exist in your game. There is no -legit- way to change them. Now, your pokemon's IV determine both the typing and the base power of Hidden Power, and since you can't legitimately change a pokemon's IV his/her/it's Hidden Power type and power are also set in stone once the pokemon is created. There are three ways that you can check your pokemon's IV to make sure it has the correct hidden power for your needs:
1- Rare Candies
With this, you just level it up to lvl 100 WITHOUT it having any battle experience or giving it any items such as Calcium, Protein, etc. Then you take the pokemon's stats and input them into a IV calculator. At lvl 100 you will get the exact IV's and thus the exact hp type and power.
2-Training
This is a much more tedius method, but for some people is the only way. Train your pokemon and keep track of EVERY EV it recieves until it reaches lvl 100 and then follow the second part of method 1.
3-Wifi
This is only available on the DS games (DPPt/HGSS). You enter a lvl 100 battle so as to get the pokemon to lvl 100 thus allowing you, again, to see what it's stats would be and then input them into a calculator.
Now with each method, please keep in mind that you also need to fill in the pokemon's nature in the calculator as well.
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EV's
Just to clear this up, EV's don't effet IV's at all. The only thing that EV's do is increase stats. (+1 to the stat for eah 4 EV's it gets in that stat up to the cap for a single stat -255 ev's or +63 to a stat-) Now, the maximum is 510 EV's total spread throughout the different stats. Now since i've said all this i'll go ahead and add that the most ev's you should put into a stat is 252 since that is the highest amount divisible by 4 that you can give without wasting any. (252/4 = 63 --- 255/4 = 63.75)
Hopefully that clears things up, and if you still have any questions there are numerous guides about all of these topics readily available. If needed, I can provide links.