hi i am a altmer (high elf) and my birthsign is the apprentice i know about the atronach, but i can't find its stone. i have 100 intelligence but i need more magicka to cast the best version of fingers of the mountain and make realy good spells can anyone help. much appriciated

Enchant some robes with Transcend. Sigil stone: Fortify Magicka 50pts.
Avoid Fortify Intelligence, because max you can obtain is 12 pts that will bring only 24 magicka more.

Download "Fingers of the Mountain fix" because the spell is well known to be bugged, the magicka cost being much higher than what it should be.

Also, if you are not Master in Destruction magic, think about some Fortify Destruction enchantements, this will decrease spells cost.

Learn to layer various lower strength buffs and effects to achieve a larger effect for less. Like using a fire effect for 3 seconds in one spell, a cold effect for 3 seconds in another spell, and a lightning effect for 3 seconds in a third spell. Then alternate between two of them depending on what you're fighting. That way you get a few extra points of constant damage. You can also use multiple fortify spells in the same way to get around skill limitations or to make the most of each point of magicka spent casting it.

You should also make use of potions. High level alchemy is almost like an instant win. Weapons enchanted with absorb magicka, or weakness to magic/element can also be useful. Just because you're a mage doesn't mean you can't use a weapon.

Ignoring those special spells entirely, and just focusing on shield enchantments (light armor) with damage shield spells, and absorb health can do wonders. You can just drain them dry as they pound on you, with damage shields making up the difference.

Summons can also be useful, especially when facing multiple opponents.

And don't ignore the value of a good damage fatigue spell. If they can't move, they can't fight.

what is the best summon spell not special onces that you get 1 a day for completing quests and should i enchant my own weapon because i am using dawnfang/dustfang at the moment.

One type of spell you should think about getting/enchanting a weapon with is Damage Strength and Damage Intelligence and Damage Agility.
Depending on what kind of enemy you are facing, you can destroy their ability to cast spells, hit you in the face with their huge sword/move at all, or pump you full of arrows.
Disinigrate Weapon should also be sought out. How can that axe-wielding Nord harm you if his axe is broken? This is especially useful when enchanted onto a bow or as a ranged spell, as you can drasticly reduce an opponent's death-dealing potential before they can even get to you. Coupled with a few summoned undead or daedra, your formerly pasty-faced mage will become a well-tanned killing machine.

As for enchanting your own weapon, it's always a good idea to enchant your own. This gives you a greater level of versatility when preparing for combat.