Ok so I am a little new to playing a magus and was wondering if you added anything to the damage rolls of his spells or if the only way to increase the spells damage is through arcanas or by leveling up in general since I know must of the spells increase the number of damage di as you level up
I can see three main ways a magus can increase the damage of his spells.
You can use SpellStrike to give yourself a change to get a critical strike on a touch range spell. Since the attack uses the weapon's critical range, you can give yourself a significant change of doubling the damage of your spell.
As far as I can tell the Magus can use metamagic feats just like a wizard can. A few of those feats increase the damage of spells like Maximize Spell that maximizes every random effect of a spell making a 10d6 fireball always deal 60 damage.
As you said the effect of most spells increases with your level. The magic missiles of a lvl 10 character can end up surprisingly potent.
No, there is no modifier you add per se. Of course there are magus arcana (empower and maximize), feats, magic items, etc. that will augment your spell damage, but if you are first level and cast a shocking grasp you get the damage it says on the tin.