Problem

You want to make fire on a part.

Solution

Use the Fire object.

local fire = Instance.new('Fire')
fire.Size = 25
fire.Parent = workspace.Torso

Discussion

Use the shortcut for creating an object within a directory by using the second argument to Instance.new. Then set the returned object’s size to 25. You may also change the base color of the Fire by using the Color property and you may also change the outer color (basically the ends of the flames), using the SecondaryColor property.