Okay so I have this trigger:
Variable - Set Random = (Random Living unit from (Any units in (Region(Position, 6.0)) owned by player Any Player matching (No Value), with at most Any Amount))
Basically, set Variable Random equal to a random living unit within a circle (radius 6) around position (position of the unit casting the spell) is what this is doing. Two things I don't get, how do unit filters work? If I exclude "self", does that mean I am not selected within this random radius? Because every time I try that, the spell always casts on myself. The second is, what does the last variable do, "any amount", you can't store more than one value in this "unit variable" can you?

This trigger also always manages to target some random thing off the map, I have no idea why. So when I take the position of the unit, it returns an overflow (error).

Not sure about what you're asking, but here's some possible answers:
In unit filters, you normally let the three pre-selected value as they are (Missiles, Dead and I-don't-remember). That means these will not be included into the trigger count.
The 'Any amount' can be set to any number. It restricts the trigger based on its value: if it'd be 5, the trigger wouldn't run as long as there is more than 5 units (that meet the UnitFilter conditions) within the region.

Did I help? :S

That makes sense, the amount part, I managed to get everything working besides effects, how do I add special explosions/effects to a map?

Do you mean explosions/effects already existing or custom ones?