I made my second character which is a mage, My major skills mage class ones except I replaced alchemy with marksman so I don't level too fast. I disabled the +5 attribute modifiers so I don't feel guilty, if it gets too hard i'll just lower the difficulty.

I have no idea how to play a mage, where do I start and how would I use all the skills?

Try joining the Mage's guild and beat it without swinging a sword or mace. Use light spells instead of torches, use absorb magic to gain an advantage in battle, and use shield spells instead of wearing armor. In time, your magic skills will raise.

Oh and start using the altars to enchant items and create custom spells once you get into the Arcane University. That will make being a mage more fun.

ha! i use a dagger or sword as i have blade skill for my war mage but no armor

Oh I almost forgot! Using bound weapons/armors works well as mage, even if you don't have a major in blade or armor.

Also, level your illusion fast. It turns out to be quite usefull after lvl 50, what with command humanoid/creature, demoralize, frenzy, and paralyze. You can also use conjuration to deal with multiple enemies. Use mysticism to spot enemies from afar.

Go back and get alchemy as a major. THEN USE IT!

...thats how you can play any character type btw

There are lots of way to play as a mage. You can use destruction to blow them apart, illusion to make them fight with each other, conjuration to summon your minions, restoration to absorb the hell out of them or fortify certain skills to make you a warrior.

I prefer Illusion and restoration method....
My weapon is a silver dai katana... I fortify my blade and strength to 100 and 2-3 power attacks would finish off my opponents...

Given your majors, you start the game with a pair of good touch destruction spells and the weak but ranged Flare spell. You can also summon a skeleton, and have a couple low level mind control spells. Get through the tutorial using those tools. Don't be afraid to pick up a dagger or staff and use it to block while you cast spells to fight, and don't be afraid to ask that skeleton to help. Get through the Mages Guild recommendations, buying what spells you can, and get into the University. Spend plenty of time there learning and training and crafting spells and practicing. There are plenty of places to fight and train right there on the Imperial Isle. When comfortable, head out to adventure.

Playing as a mage is fun.

1st. join the Mages Guild, you get access to a whole bunch of spells and free loot.

2nd. Do all the reccomendation quests, The one in Cheydinhal may give you the most trouble but that one can be easily solved.

3rd. Once you got to the Arcane University you can create custom spells, although you need the regular spell 1st. You can't create an Invisibility Spell if you don't have a regular Invisibility spell.

4th. The rest is up to you, good luck.

Thank you for the help
What is the third stat I should invest my points in other than intelligence and willpower?

I want my mage to use destruction to kill enemies and restoration to heal myself, are specializing in these 2 skills viable or do I need to practice in other schools of magic?

Personality governs Illusion. So if you are truly going pure mage, that would be the best 3rd attribute. However, depending on whether you loot a lot, you could focus on Strength to carry more. Or you could get more Endurance to get more HP per level up.

You are correct that intelligence and willpower are key. You could pick any of the several below as a third depending on what you want:
Agility - if you want to get good with that bow, this will help.
Strength - if you want to use a blade effectively and/or carry around more loot, this can help. If your mage doesn't use a blade, maybe not. The loot is not a big deal since mages can mage potions and enchant rings - both are very profitable low weight pastimes.
Endurance - more hit points. Only important if you expect to get hurt. My mage (mystic archer) safely ignores it.
Speed - if you want to avoid damage by running. My mage prefers to simply disappear and walk, so speed is not important..

Your second question: There are many ways for a mage to successfully deal with foes. Focusing on destruction for killing and resto for healing is certainly one of those ways and it will work fine. That said, as a mage, I recommend getting comfortable with the capabilities of all the schools unless for RP reasons you want to limit things. My mystic archer for example remains very poor (for RP reasons) at using fire, frost and shock.

After Master Alchemy (Attainable in about an hour or less of game time) nothing else matters.