Someone tell me please and a few tips I should know

I dunno... I sneak when I'm close to enemies no matter what the build, so Sneak always increases. But I will say this, get that Quiet Casting perk off the Illusion tree ASAP. If not, you might as well find a way to delete the Sneak tree.

I could be wrong but I don't think you can get sneak attack bonuses with Destruction magic--if that's something you had in mind.

If you want to be mage with great sneak ability in order to be as non-combative as possible then I would imagine it'd be a lot of fun. But I like to kill enemies in Skyrim. It's fun.

Yes it really is, I played as a sneaky witch once and she was really fun.

Use illusion to disorient targets (like pitting them against eachother) then destruction to pick them off.

By using sneak you can make effective use of Illusion spells. It also allows you to setup runes and choose where to place a conjured creature. I played one illusionist pacifist build that sneak was essential too.

It is it's own definite style of play. Whether it's fun depends entirely on the kind of things you look to get from the game. For a person that gets bored easily without a good deal of action, this type of character may not deliver. It is more of a chess players/strategy game player minded approach. It can be executed with a sort of swift efficiency, especially after you've done it often, but its very nature rewards patience and persistence. This is not to say you won't fight and/or run, just that if you are pulling off the tricks of the trade effectively, you may discover your combat skill lagging as you don't wind up using it.

There are variations on the theme. A shadow archer, primarily conjuring, destruction, or even dual wielding killer (or varied combination thereof), all are valid builds. Illusion may be argued to be of use to any variety of stealth mage. It depends on your tastes, but they are definitely fun to play. People have complained that they are too powerful, but that complaint seems to be more general than one playstyle. It's just a simple fact of what you are creating that you wind up tapping the power of magic while negating the weakness of the classic pure mage (weakness to melee) with the avoidance of stealth.

Personally, I think it's merely a side effect of this character type's play forcing more careful, deliberate strategic thinking. Some do this anyway even with their warrior characters, but for those that just think of such as "dash and smash" and no deeper than that, playing this kind of character may be the first time they wind up slowing down and thinking a few steps ahead. In other words, I believe the power is in strategic play, not the tools used. Consider your adversaries and use tactics like divide and conquer, terrain advantage, etc. and warriors can seem just as overpowered without super enchanting or smithing.

I definitely recommend it. Just don't let it frustrate at lower levels of sneak that it may not seem to work. It doesn't take long before the skill raises way fast. Early on, not wearing any armor and moving as slowly as possible helps get the most out of what skill you do have. A very useful tactic, shooting arrows from behind cover or in deep shadow against a wall or down a hall to send enemies looking one way while you go another (or step up behind them to backstab). Have fun with it.