Welding Tips and Tricks

I'm new to welding and desire the ability to weld thin sheetmetal for automotive projects and repairs. I have a Miller 220 multimatic machine and in the settings I cannot set the amperage below 25amps. The machine has "smart settings" and for some reason I cannot enter manual settings so I can practice on sheetmetal thinner than 20 gauge.
May I request some insight on what I need to do to manually set this machine up for the material I want to work with?