I'm completely new to Microlite20, and to the D20 system in general. What, if anything, do I need to roll to cast a spell in Microlite20? The rules say "Magic Attack Bonus = MIND + level". Is that required for all spells, or just spells that directly do damage? And what about spells that don't do damage at all, like Light?
In Microlite20 and d20, the caster rolls dice if the quantity of success is unknown (damage inflicted, amount conjured), or if a spell must be aimed at a target. All other spells just succeed. Even if the caster doesn't roll dice, some spells are resisted by the target, who would need to roll higher than (10 + Caster level + Caster MIND bonus) on a d20 to succeed in resisting.
The magic attack bonus would be used when a spell is used to attack a target, just as a melee attack bonus would be used if a weapon is used to attack a target. In the case of spells that do not cause damage like a weapon, the caster may still need to touch the target, or aim a ray at the target, and the target may resist by dodging or trying to interpose another object.
With your example, the "Light" spell aimed at an inert object (a tree, a rock, an enemy bound hand and foot) doesn't require a roll determine success. Someone trying to run away in the dark and would rather not be a beacon leading the chase, that target would resist and require a roll for success. It's up to the game master to decide what would be appropriate and interesting for the story.
I'm not as familiar with Microlite20, but I know d20 pretty well. Based on my brief glance at the Microlite20 Gold Edition (V10) rules, the Magic attack bonus is the modifier you add to a 1d20 for the attack roll for magic spells which require such a thing. The spell descriptors will state whether an attack roll is needed.