I have decided to start a new character and it's a magic based one, but here's the kicker, I have never used magic before (apart from flare and restoration). I am going to be a Altmer, I have already have an Imperial save, as Breton's look the same I'm not interested. I'm thinking as it's my first go at a proper magical character I'm not going hardcoe like a mage, but maybe something in between. This is the build I was thinking and suggestions or advice is welcome.

RACE: Altmer
BIRTH SIGN: I have a few questions, as a High Elf I have a 25 weakness to fire, shock and frost. Born under the Apprentice I have a 100% weakness to magic. So how will this affect my character? Is it better to be born under The Mage?

CUSTOM CLASS:

Magic
Intelligence and Willpower

Alchemy
Alteration
Blade
Destruction
Mercantile
Mysticism
Restoration

The reason for blade is for those close encounters. Also I wasn't sure about Alteration, but not sure what to replace it with.

You will get several different views on this one. Pending on your playstyle, the App BS makes for a weak character against casters, for sure. It will take some strategy, to handle those. A first time Mage might consider using the Mage BS. But if your up for the challenge, the App is OK. I would read the USEP, on magic stacking and chain spells. An understanding of how they work, will help a lot. Chains are good for those that have a limited magicka pool. Having a maxed out pool is not a necessity, by any means, but it does have it's advantages.

I think Intel and Wis, are good. You should Max them out as soon as possible. In that order. I have a current Alt / App character. That has Intel / Luck, as favs. I'm one of those Pro Luck nuts.

I would think about replacing Alc and Merc, with Illusion and Conjuration. Ill and Con are arguably the two most powerful schools of magic. Followed by Restoration and then Destruction. Being able to conjure a creature to fight for you, and turning invisible. is an easy way to handle a lot of enemies, like other mages. It works well, until you get the hang of multiple spellcasting.

Blade is fine,but not needed. I opt for it over Alc. It helped me cap my character and control my leveling a little. A high magicka pool, doesnt have a large need for potions. Or a need for armor or weapons ether. Do NOT wear armor. It makes your spells less effective. The threads 'Daggers are Deadly' gives examples of enchanted daggers, that are very powerful, and don't require a high Str or high Blade skill.

Good Luck

As a mage, you are wise to consider one of the three magic birthsigns. Here are some thoughts on the three as they relate to you playing an Altmer. I am taking strongly into consideration that you say you are inexperienced playing a mage.

Apprentice. Avoid it. It is wonderful for players who are trying to build a 'glass cannon' that is both very powerful and very vulnerable. A delightful challenge to play for experienced mage players. The hazards are magnified with an Altmer.

Atronach. Avoid it. An Altmer Atronach is arguably the strongest mage of all. This birthsign can completely negate the Altmer's weakness to the elements with a few enchanted items. The result is a very safe mage with a boatload of magicka. So what is not to like? The stunted magicka. Specifically, experienced players have no trouble with it and it is easy to deal with; however, those new to playing mages are likely to become frustrated. Although I highly recommend the Altmer Atronach, I strongly suggest you play other mages first and get comfortable with them.

Mage. Pick this. It is simple, safe and carries no drawbacks. The somewhat smaller boost to magicka that it provides is well worth avoiding the downsides of the other two birthsigns for a novice mage player.

Although you did not mention it, you might consider a Breton with the mage birthsign. This is not only a wonderfully safe and powerful mage for any player, it is extremely forgiving for those new to playing mages.

For skills just pick the 6 magic schools as well as heavy armor. That way you have all the magical ability of a pure mage, but can also survive better and are more suited to using on touch spells (which are cheaper than on target) than a regular unarmored pure mage. Only problem with wearing armor is your spell effectiveness will not be 100%, but having it as a major and constantly using it ensures that the penalty will be 5% at most, although the added protection will more than make up for it.

A small penalty to spell effectiveness wont make any difference at high levels when you can cast some ridiculously powerful spells.