So if I wanted to raise my destruction alone to 100 and leave all other skills alone, what level would I be? Is there a system to figure this out? Cause since Destruction is so weak I was thinking to make a Mage with Destruction/Smithing or Alteration/Enchanting, and only train those skills as to keep my level lower. So yeah, anyone have any idea how to figure out what level a character would be?

Try this: http://skyrimcalculator.com/#47679
It might give you want you want.

Ehh that doesn't do what I'm asking for.

How to figure it out? Make a toon and do it?

Well, when I chose every Destruction perk it bumped the Destruction up to 100 and estimated the character LVL at 18, but I don't know if that's a realistic estimate.

http://www.uesp.net/wiki/Skyrim:Leveling
The formula for character leveling is as follows:
Character XP gained = Skill level acquired
Example: Training alchemy from 20 to 21 gives 21 Character XP points
XP required to level up your character = Current level * 25 + 75
Example: 100 XP is required to advance from level 1 to level 2, and 1300 XP is required to advance from level 49 to 50. This is consistent across all levels. (70→71 follows the same formula as 3→4)

Buck, it just assigns the level to how many perks you have chosen.

And thanks svann, that's exactly what I needed.

Yeah, I've never really messed with it I just figured it worked in 2 directions by knowing what level you would be at skill level 100.

There are some calculators out there that let you choose skill values then tell you what level you'll be. I don't have any links but you should find them relatively easily on google.