How to make my fireball disappear if it hits a wall?

Hey!

BasePart's have a .Touched event you can use to see when it touches anything. To make it disappear, you can destroy your fireball if the event is ever fired. An example is shown below...

fireball.Touched:Connect (function(t) -- t = The part that just touched your part
	fireball:Destroy()
end)

That is pretty much the extent to how .Touched works! As you can see, it also passes a parameter, this parameter being whatever part had just started touching your fireball and caused the event to fire. You can use this to specify what parts you want to ignore from destroying your fireball. In the example below, I inserted an if statement making your fireball ignore any parts named "IgnorePart" from allowing the fireball to get destroyed.

fireball.Touched:Connect (function(t)
	if t.Name == "IgnorePart" then
		t:Destroy()
	end
end)

Hope this helped! If it did, please remember to mark this answer.