Welding wire ?

I run my gas at about 10#

the wire size is according to the material thickness and the gap you are filling for thin shhet metal .023 is fine -when you get into heavy metal (like subframe connectors .033 or .040 is better)
when you have the proper heat and wire speed the sound will be like a steady sizzle (you will know it when you hear it)
also on thin metal drag the gun and heavy metal push the gun