Now you want to be a mage. You'll need to train up your magic schools. There is only one way to do this (outside of trainers, which you pay for), and that is spamming spells from each school.
The schools of magic in Oblivion are: Alteration, Conjuration, Destruction, Illusion, Mysticism and Restoration. Your most useful schools will be (obviously) Destruction and Restoration. The other schools will also have useful spells for other things, so training them all up to at least 25 (apprentice) is highly recommended.
To train up each school, you'll need a cheap (magicka-wise), on self spell. The best way to do that is different for each school, for you do not want to injure yourself with on self destruction spells. The best way to do this if you don't want to join the Mage Guild and work your way to be able to access the Universities spellmaking/enchanting altars is to get the Wizard's Tower plugin, which is Frostcrag Spire. Essentially, Frostcrag Spire is a personal mage University with a spellmaking and enchanting altars. All you'll need to do after 'finding' it is to get the magetallow candles from Aurelinwae at the Mystic Emporium in the Imperial City Marketplace. Be warned though, there is a bug if you also have the Fighter's Stronghold plugin, where the Emporium is permanently locked. There is a workaround, and if you don't want to incur a bounty by picking the lock yourself, the console is your friend again. Bring up the console, click on the door and then type 'unlock'. Then when you go inside, go upstairs and do the same to the bedroom door. No bounty and the Emporium will be open for business. Another quirk is, after you do this, you can pick up anything on the shelves and not be stealing. So pick up whatever you want and sell it back to them for more gold if you like.
Now, the best way to get the best price on the candles is to chat with Aurelinwae and make her like you more. Once her disposition is maxed (hopefully around 70 or so) you can then click to trade with her and haggle first. Since she has a low mercantile skill, you can haggle her down some on the candles and get them for just under 3k gold each.
Now, take the candles to Frostcrag Spire and put them on the altars. Now you can make cheap training spells. To do this, you must first have a spell that you can use. You can purchase spells from almost any mage in any guild hall or from alchemists. Restoration spells can be bought from a healer in any temple in any city.
For Alteration, all you need is the Open Very Easy Lock spell, purchasable from Calendil in the Mystic Emporium. Then all you need to do is go to the locked barrel on the Waterfront (it's the only locked barrel out there, sitting next to Methredil's house) Then just spam the spell on the barrel. Spam it until you run out of magicka and then drink a Restore Magicka potion or use a Welkynd stone (found in any Ayleid ruin, but they do not respawn, so use them carefully) or simply wait for your magicka to regenerate itself (Unless you chose to be born under the Atronach, in which case either potions or Welkynd stones will need to be used). You want your Alteration at least 50 to be able to use Feather spells. Feather spells will lighten your encumbrance to allow you to carry more. See the UESP site I linked to in the first post on this, and any other subject having to do with the game.
For Conjuration, get the Bound Dagger spell from Alberic Litte in Chorrol Mage Guild. Then go to your spellmaking altar and make a training spell with a duration of one second (the smallest you can make). Then just spam this spell until your Conjuration is as high as you want. This is good if you want a set of Bound Armor, for there is a glitch where you can drop the bound armor before the timer runs out and it disappears. More on this later. But if you want a set of zero weight armor/weapons, then you want your Conjuration to be at least 50-75. One note. Runestones (the tall stones with green glowing runes on them) will also level your conjuration.
For Destruction, if you did not choose it as a major skill, you will need the Minor Enervation spell from Druja in the Skingrad Mage Guild. This spell only drains fatigue (which will then regenerate). Go to your spellmaking altar and make the cheapest one. Make sure you set it to be on self since the default will be on touch. Then spam this spell until your Destruction is as high as you want it.
For Illusion, you can buy Starlight from Calendil (or Edgar Vautrine of Edgar's Discount Spells) and make a short duration spell with it to spam endlessly until your Illusion is as high as you want it.
For Mysticism you need Minor Detect Life. You can get this from Calendil or Edgar Vautrine. You may also use Minor Dispel (just make sure you make this one on self). Again, spam, spam, spam until you're satisfied.
For Restoration, get Fortify Fatigue from Edgar Vautrine and make an on self, short duration training spell. Restoration is the hardest school to level because it takes a lot of casts to level. So I suggest making Restoration a major skill so that it levels faster. In fact, I suggest making Restoration and Destruction major skills since both are very useful. I'd get your Restoration to at least 50 so that you can use Cure Disease on yourself. In Oblivion you can catch diseased from most creatures and you can catch Porphyric Hemophilia from vampires (which will turn you into a vampire if you don't cure it and you sleep for three occasions.) Having Cure Disease as a spell means that you don't have to lug around Cure Disease potions or mandrake roots to chew if you catch something.
Anyway, this is how you would want to train up your schools of magic really fast.