can you dual class a straight fighter to a mage? And if so, how? Because I am not able to do it.

Yes you can.
The rquirements are :

1. Your character must be human

2.The fighter has to have a strength of 15 and an intelligence of 17.

3. Must be at least level 2 in fighter class.

To be a little bit more precise, your Fighter must have at least 15 STR and 17 INT. These must be your real stats: Trying to fool the game by drinking Potions of Strength and Genius will not work.

There are also other stats that you need if you want to Dual to a Specialist Mage; as I recall, Necromancers need STR (or was it WIS?), Illusionists need DEX, etc. Can't remember the specifics.