A 75/25 mix of CO2 and argon is your basic shielding gas for general mild steel hard wire MIG welding. I would stay away from straight CO2. There are other tri-mixes that may show a bit of an improvement in some applications but the 75/25 will handle probably 95% of the general flat and out of position welding for most folks. I don't know if you have the Airgas welding chain there but they call their tri-mix Stargon. Less spatter and a bit hotter puddle and cleaner running should you ever get in to spray arc welding. But spray arc is usually for thicker stuff, in the 3/8" to 1/2" and up.
If you plan to weld things like aluminum or stainless then you certainly need shielding gas like straight argon but that welding process is not what the majority of folks are doing in the home garage. Unlike Rojo, who seems to be a wizard with the aluminum but that's mostly TIG welding, I believe. He runs a magnificent bead from what I've seen.