I'm a metal worker so here's mytupence worth, as suggested clean off the paint make a good contact for the earth under the body somwhere, try to discover what thickness you are welding before you start anything under 3mm thickness will be difficult for a novice using 3.2 welding rods, if like most of the trailors I've seen these days it will be 18 gauge mild steel or lighter thats 1.024mm in new money so pretty thin if so you will need a 2.6mm rod or if you can get smaller all the better.
practice on a bit of scrap the same thickness, when you strike the arc the rod should be no more than the thickness of itself away from what you are welding if you do that and it burns a hole turn down the amps, it's not exact but try holding the rod at 45 degrees to the work you are trying to weld. thats the easiest way to get the hang of it when you get better you can change the angle to direct the heat,
if you are welding 2 different thickness'es say 5mm on the body frame and 2mm on the sides hold 70% of the rod towards the thicker metal and 30% towards the thinner metal you can play with these angles when you get better.
when you are welding slow down it is a lot slower than you think, you will see a small bubble this is the slag whick will come off afterwards but the most important thing you need to watch out for is just behind the slag bubble you will see a molten pool you want it to look like loads of CCCCCCCCCCCCCCCCCCCCCCCCCCCCCC closely together if it's a load of <<<<<<<<<<<<<<< you are either going to fast or are too far away.
if you see the slag lifting away from the weld as you continue welding rejoice, that's what you want, in an ideal world, ie clean non rusted metal thickness/angle of rod correct, you would expect to see the slag lift up in a curve behind you as you weld, if you don't see it don't beat yourself up it will bang off with a hammer if it's stuborn use a hammer and chisel let it cool down wire brush it, see if you can get a bit of Galvafroid paint it's very expensive but really is excellent paint
best of luck
Regards
D