110 is fine for sheet metal, but get 220 if you think you'll ever want to weld anything over 1/8" or so.
Sheet metal is tricky. It's almost impossible to weld a continuous bead, and if you could you'd warp the metal a bunch anyway. Use the smallest dia. wire; the welder will likely have a driving wheel with a nominal 0.025" wire rating, but I've only ever found 0.023" or 0.24" wire. This can cause the driving wheel to slip and stall the feed. I bought a large (10lb?) spool and I think it causes the feed to hang, so I'd start with a 1lb spool, and make sure you use the correct drive wheel and sheath--the wound wire 'tube' to the gun--and copper tip so that you maintain contact from the welder to the wire. You usually want about 1/8-1/4" inch of wire sticking out of the tip. Make sure the line from the welder to the gun is as straight as possible; tight loops or kinks can lead to stalling the feed. Make sure metal is as clean and grease-free as possible. For butt welds, you will want to 'skip weld;' i.e. do a tack weld, skip a few inches, do another tack weld, etc. then come back and fill the gaps when things cool. Use compressed air to cool the weld and surrounding metal after each tack. Get a good grinder (the HF ones have worked OK for me). You'll definitely want gas shielding, the flux core wires are usually 0.035" or thicker.
There's three main factors in the weld: 1) feed speed, 2) voltage and 3) distance from the wire to the target. The more the voltage, the faster the wire has to be fed to keep the 'flame' from sputtering. With a 110V welder, the fastest wire speed and highest should weld up to 1/8"; dial down from there. The closer you can hold the wire to the metal without touching will create the hottest flame. Get some comparable metal and practice until you get the hang of it. Most welders have a recommended setting for various metal thickness, but you'll need to experiment. A quick search produced:
https://www.millerwelds.com/resources/article-library/miggmaw-101-setting-the-correct-parameters
For plug or 'rosette' welds, consider getting one of the compressed air-activated punches (they usually also have a fitting for flanging edges; very handy for overlapped welds). I've never determined for sure whether you should start the weld from the edge of the hole or the center; but starting from the center allows the metal to be pre-heated at the edge of the hole, which is where the metal gets bonded.