Pretty simple. If you make a tool and make it so that each time someone clicks, you instance a part (and then change its propteries to mimic a snowball)- and then give it a direction.
local direction = (mouse.Hit.p - handle.Attachment.WorldPosition).Unit
snowBall.CFrame = CFrame.new(handle.Attachment.WorldPosition, handle.Attachment.WorldPosition + direction)
Then destroy this snowBall upon it touching something, and simulataneously play a particleEmitter that looks likes it got ‘splashed’.
Lastly, as for the trail, in the handle (the actual snowball). Put additionally two more attachments (0 and 1). Which your trail can refer to. And so when the snowball has been thrown, you enable the trail.
I recommend that you check out fastCast, where they make everything EASIER for you. It is an awesome module and I recommend every noobish pleb like myself to use it.