How to be a pure mage.

Step one: 
Making your character
Make your character and give it a cool name.  Finish tutorial island with all stats 1 if possible.

Step two:
Gathering supplies
After finishing tutorial island its time to grab supplies.  Run to goblin house across the bridge from lumbridge.  Grab as many bones as your inventory can hold (28).  Run to the grand exchange (use your world map if you can't find it) and sell all you bones.  It should come to a total of 2.8k.  Purchase a air staff and with the remaining money purchase as many mind runes as you can.  I wouldn't buy wizard robes yet but when you have some extra money get them.

Step three:
Note:  Every time you get a new spell you have a choice of switching to that.
Mage level 1~10
Chickens.  Pick up feathers and bones to sell.
Mage level 11~20
Cows.  Pick up the cowhides.  Cowhides can be tanned if you have some base cash.  You can make some extra profit if you tan them.  The tanner is located in Al-Khardi.
Mage level 21~30
You can either start mageing barbarians or you can cast curse on level 1 rats.  Do not kill them!  I love the cursing rats because of the good xp and they can't hurt you.  Please note that you must be wearing full armor so u splash.
Mage level 31~50
You can can either continue cursing rats or start mageing guards.
Mage level 51-70
Moss giants.  Be sure to use safe zones.
Mage level 71-99
You can cast high alchemy for tons of exp quick or you can combat lessers, greaters, or hellhounds.  I would recommend high alchemy because of the quick xp.  And you can buy rune warhammers very, very cheap from other players.  Then high alch for some profit.  Make sure to use fire staff.
Best possible mage offense
Hat:  runecrafter hat
Cape: colored cape  
Top: Combat top  (if you are pure mage (recommended) use regular wizard robes)
Legs: combat legs  (if you are pure mage (recommended) use regular wizard robes)
Main hand: Any magic staff
Off hand: Anti dragon shield
Feet: leather boots
Hands: leather gloves