I may be in the minority, given the recommendations above, but I'd recommend buying a stick welder, not a mig if you're a beginner. A mig is easier to "glue" things together with for a beginner, but does not result in as good of welds. They look good, but have less strength. You will learn a lot more and get much better structural welds with a stick welder.
I'd buy an older Marquette, Fornery, or tombstone welder, if you can get ac/dc it will be a better machine and more versatile. I see these all the time for $50 to $150 on cl or at estate sales. I bought one a couple of years ago (an AC/DC Forney) for $75 including good leads, a helmet, rods, gloves and chipping hammer. The one I had before it was a $40 yard sale Marquette, again came with helmet and rods.
I also recently bought a Miller Dialarc 250 with a tig unit for $350. Now, that's a heavy unit, weighs about 600 pounds and has a LOT of capability if you have the space for it. The newer inverter units are much lighter, but they're a lot more expensive for the same capability. Used older units are a good buy because of people upgrading or older guys giving up their tools.
I also own a small 110 volt mig, like most homeowners buy to weld with. It's a pain to use, and has very low capability. I was welding some rebar column cages the other day, just tacking them together before tying the hoops to hold them for real, and it kept kicking off because of the light duty cycle. Plus, harder to get any kind of penetrating weld with it. Made me remember why I sold the last one of these I owned a few years ago.... But without a shop to set up my better welders in, it was better than nothing. Can't wait until I get the shop finished so I get back to full capability.