Mario --
Welding has very little to do with the "machine" and very much to do with the operator... It's like a golf swing -- if you suck -- no amount of money spent on clubs will help. If you're good - you can hit any club. Having said that there are a few things that you can do to help.
CLEAN CLEAN CLEAN not only the side you can see - but the back side too! CLEAN!
Fit up is critical! Poor fitting pieces are a nightmare and the increase in skill required to weld them increases exponentially!
When tacking - I snip the wire at an angle - like 45* This make a good initial contact and actually takes less voltage to get the weld started. It also makes the tack with pure clean fill metal. I snip - tack - snip - tack.... Wire is CHEAP. Watch your "stick out" - tacking is best done (for me) with a little shorter stick out than if I'm running a bead...
I like to tack CORNERS first. Heat runs to the corners... and by the time you get there - they're already way hotter and tend to just blow.... So I go all the corners - then the middle of the piece centered between the corners so the metal doesn't start to bow or warp.... cooling as I go -- then in between each of those tacks until I'm down to an inch between tacks or less....
TACKING is one of those things where you really really need to be able to SEE!! If you can't see your work and see exactly where your tack is going and what is happening to the metal then it's hard to do. Once you're really good at welding - then you don't even need to look - but until then you need to be able to really see your weld! Play with the darkness of your helmet. Too dark and you can't see and too light and you're seeing spots all day! LOL.
Practice these settings on scrap at the wire feed and settings you're going to use to do your work.
I usually angle my MIG wire at the BIG PIECE of the work -- rather than trying to aim for the gap... the big piece is more "solid" and usually your GROUND is connected to the big work... so your tack will get off quicker -- and the puddle will gap over to the edge of the filler piece... if that makes sense...
I MIG Tack and TIG to fill the gaps -- because grinding TIG fill rod is far easier! But TIG takes far more skill so just figure you'll have to be careful grinding...
TIG can also be hammer and dollied to flatten and control the warpage and the amount of grinding required.... and MIG wire is just too hard to do that with.
Tacking is all about heat control and most "newbs" I've tried to teach are just on the weld too long so they burn thru.... It's kind of a timing issue... and just takes a bit to master but it's not hard that's for sure. If I can do it - you sure as heck can!