Most useful destruction energy type and managing perks

My mage knowledge is mostly second hand but I can give out these pointers so far:

• You can not get a higher tier perk with out getting the lower ones. For example you can do nothing in the destruction tree with out starting with the Novice perk. This applies for all of them as well.
• There is no end all be all elemental skill. A lot of people find Electrical the most useful simply cause it instantly strikes what is under your target, you do not have to lead your target. But Ice works against fire based dragons (and I've found it very useful against giants), fire works against both kinds of trolls, all of the undead, and electrical works good against mages. If you are trying to be an offensive mage you will really want to go with all of the elements in the long run so you are never stuck with just one choice.
• Impact is pretty much a Must Have. When my brother got this he went from having to do some fancy kiting of enemies to being able to constantly stumble them. This is -not- the win button, but it definitely puts you on a bit more even footing than you are with out it.
• When considering Conjuration keep in mind that the max level summons, known as thralls, are permanent until they die. You will need to have enough mana on hand to cast them, but it may not be worth while to put perks in here aside from those that allow you access to have 2 summons at once.

I hope this high level overview has been helpful and addressed your questions. Any more concerns ask in the comments and I will try and add in more information as I currently understand it.

If you are going pure mage: Destro, be sure to spread out your perks... Do not focus on one specific type of cast (IE just fire). There are several creatures in the skyrim world that have immunities and resistances to each of those: Frost, Fire, and electric.

Be sure to get the stun perk and a dual casting perk in which ever specialization you see fit. My Recommendation: Fire, as it does great damage (Ranking from lowest to highest amount of damage done: Frost, Fire, electric), doesn't waste all of your magika instantly, and stuns your target for easier kiting when combined L&R hand. (REMEMBER: you must have flame or your dual specced casting ability in both your left and right hand, then cast at the same time for even more damage via those perks).

Even though you might not put points in conjuration: if you are having a tough fight, call on a atronach or your partner to tank or pull 'aggro' off of you, while you nuke your opponent.

As a mage, you will have to use your magika wisely or else you will be kiting enemies for hours. My recommendation is to boost your magika to around 300-310 and then focus your gear on magika regen. A big pool of magika will be pointless if you cannot rebuild it fast enough.

Hope this helps.

EDIT:

You cannot skip items in your available perks path. You must unlock the pervious one to it, for the next one to be available. But in circular perk paths - such as blacksmithing - you can choose to go one way as appose to the other (left / right).

You can always save your perks for later, but be sure to grab the health / stam / magika bonus as soon as possible

It's probably a bug, but the Illusion perk "Aspect of Terror" makes fire spells do +10 points of damage. So for example, my character with Aspect of Terror and the first level of Augmented Flames (1/2) does 41 points of damage with his Firebolt spell and 20 (!) points of damage with the formerly weenie Flames spell. That's right, 20 points of damage per second from Flames.

I guess Fire spells have a Terror quality to them whether you have the Searing Flames perk or not, which is why Aspect of Terror affects them, but I'm sure this is a bug. A very fun bug though =) this is still current in version 1.5.