Agree with most of what has been said so far.
+99 on MP&C (Robert McCartney). Read through the linked thread. LOTS of good info and tips there. note: it's a LONG thread. Well worth the reading though.
Good fit up is important.
A backing bar or "chill bar" is sometimes helpful if you have access to the back of the weld. I prefer copper as it conducts heat very well and a steel weld won't stick to it if you melt through your sheet metal. Still makes a mess if you do blow through your steel sheet to the copper backing. Aluminum can work too, just not quite as thermally conductive as copper. You can buy or make a 'welding spoon'.
Lots of weld 'dots', not a continuous weld bead when welding thin sheet metal. Make a weld dot, move several inches away to another spot and make another weld dot. Grind away some of the weld dot if it is too high/proud of the surface (don't overheat and warp your sheet metal while grinding!). Planish the weld dots and try to keep the original shape of your workpiece as you go, don't try to weld the whole thing and then try and beat it back into shape at the end. Repeat the weld-cool-grind-planish until the entire joint/seam is fully connected by weld dots and fully welded
Each weld dot still has to be a GOOD weld. Adequate/correct fusion/penetration of the weld dot fully into the workpiece.
It's a different mindset from running a nice long weld bead on a seam/joint on thicker material. You need to adjust your overall pacing and realize that although you might be able to weld a bead the entire seam long in 5 minutes or less, when doing sheet metal the overall pace is more like 'Make a few well spaced weld dots and let them cool. Grind them down and then planish them. Repeat for the next hour or so.'
As mentioned, ANY weld will shrink as it cools (hot molten metal has a bigger volume than room temp metal, so the weld bead and the surrounding HAZ will shrink and warp/distort as they cool after welding. Brazing does it too, just a slightly lower amount of shrinkage usually because the temperatures are lower than welding although being ham-handed with the torch can make the shrinkage/warpage worse than welding).
Use C25 gas and not 100% CO2. Pure CO2 runs a 'hotter' bead (more thermal energy into the weld not actually a hotter temperature arc or puddle) than C25.
.025 solid wire is the usual go-to for welding thin stuff. Bigger wire can work, but it's a different dance.
Clamp the workpieces well and tack them in several places along the joint/seam. There are small panel clamps that can be used to hold the joint/seam together, sometimes they are helpful and sometimes they are more trouble to deal with.
If you started on one end of the joint/seam and tried to weld the length without clamping and doing some tack welds at least near the ends of the joint/seam, as you progressed along the joint/seam the thermal expansion WILL make your panel gap grow. A LOT.
Welding thin sheets together is a dance of weld penetration but not blowing a hole or holes through your panel, while managing thermal distortion. You HAVE to place your weld dots relatively far apart to not get too much heat into one section of the panels at a time.
Oh, and practice. Practice some more.
Do you have any pics of what you are trying to do? Pics might help someone here to give you some more hints or tips or critiques.
