Not sure if you have ever welded..., or not, but here are a few things I learned with MIG. In addition to a helmet and some form of a jacket a Pro Welder friend told me to wear ear plugs. That keeps any sparks from striking your ear drum! Make sure the metal is CLEAN. Have a good solid ground. If it is a 120 volt welder avoid extension cords and if you need one make sure it is 12 gauge and only as long as needed.
Sheet metal is a pain because you are likely replacing rusted areas. Anywhere the metal has thinned it is suspect to blowing a hole rather than fusing the steel. For me that is (notice I didn't say 'was') the greatest difficulty. Putting a piece of copper tightly behind those areas (potential holes) helps as it continues the continuity of current and also helps to draw heat. If you feel the gun pushing away from you, you have too much wire feeding. Use recommendation s BUT wire feed speed and current levels are something you perfect over time.
Be prepared to GRIND. MIG welding is a constant additive method. Other forms (gas, TIG) you add metal at your choosing. But MIG is like Arc welding, you aren't welding unless you are adding.
What Welder did you get? 20 years ago my wife was in Costco and they had a floor model $450+ Lincoln MIG PAK 10 on sale for $250. It has been a workhorse for me. Just try and stay within the duty rate cycle. Thankfully I have never tripped the internal core breaker. The other is if you are using shielding gas over flux core make sure you turn off the bottle. Nothing is more frustrating than days later finding you have leaked $35 of gas from a new bottle!