1st time welder

Definately go back and get that auto-darkening helmet.

The welding shop will know what the standard mix is- 75/25 Argon and Co2.

I would suggest NOT spending a bunch of time with the flux core.
Maybe three or four welds, just to get a feel for the equipment.
Once you get the gas going, it's night and day.
You can see the puddle, and it's less smoky, poppy and hardly any slag.
Plus the weld is nicer and stronger.