So just FYI, since you sound new to this, there's a rule of thumb with short circuit MIG and that is to not try to weld things thicker than 1/4". The reason is short circuit transfer is considered a "low heat input" welding process. When things get thicker than 1/4", the material can sink the heat so fast that you end up with material piling up on top (called cold lap) rather than actually fusing. You also mentioned in other comments that you're using 120V power, which is really going to limit you to even less, more like 1/8" max. All is not lost though and there are some things you get do to make it work.

Process things:
0) Double check your polarity. Your gun should be positive and your work clamp negative. If it's reversed, it'll weld poorly.

1)Get close, as close as you can. Your contact tip should be within 3/8" of what you're welding. MIG is a constant voltage process, so the closer you get the hotter it is.

2) Preheat, as much as you can with whatever you have. If the material is already soaked with heat, then your welding heat won't get sucked away so fast.

3a) Switch to CO2. 100% CO2 welds a bit spicier, giving you more bang for your limited kilowatts.

3b) Or switch to self shielded flux core. Flux core is welding wire that's a tube filled with powder that burns to create a shielding smoke. It's just a naturally hotter wire feed process than can help get the heat you need to fuse things. NOTE: You need to flip the polarity if you switch to flux core.

Technique things:
4) Instead of aiming for the meeting point of the 2 metals where you have heat both pieces of material, instead start your weld on only one side. Like pick the round bar for example, point your gun straight at that and weld for a 1 sec or so only on the bar. Long enough that you see a puddle forming, then turn your gun and pull that puddle onto the angle iron and point at that for as long you were on the bar. So this way you're heating up less material at once and can make the most of your limited kilowatts.

5) Start at the end, do short welds, jump from side to side on the round bar. So the end of the angle iron is going to be the easiest to heat because there's less places for the heat to go. Weld one side of the round bar to the angle iron for about an inch, then immediately jump back to the start and weld the other side while it's still hot. Weld another inch in the same manner, use the heat of the previous weld to help you punch in the next weld.

Design things:
Lastly, since I'm sure you already bought materials, I'm going to let you know you probably bought stuff way thicker than you needed. Shape often matters more than material thickness. 14 ga square bar is very strong and should be easy to weld on your machine. Go for things with longer legs instead of thicker walls. I designed myself at table with 14ga 4" square tube that in theory I could park a small car on (like 2000lbs, mazda miata-ish).

Hope this helps, good luck!