Things you'll need:
1. A tool to clean the spatter from the inside of the nozzle cup. You can use either a 1/2-inch diameter wire plumber's brush (used for cleaning the inside of copper fittings prior to soldering); or a special MIG nozzle reaming tool. They both do the same job. The wire brush is cheaper but will need to be replaced when it wears. The nozzle reamer will last at least 20x longer as it really doesn't wear.
2. Buy some 90/45 degree welding magnets. Invaluable for holding metal in position for welding.
3. "Nozzle goo." About $5.00 for a jar. Dip the hot nozzle cup in the goo - keeps spatter down and from collecting on the inside of the nozzle cup.
4. MIG welding pliers (Welper) . Cut mig wire, spatter removal, wire pull out from tip, use them for changing tips, etc. A clip-on belt holster (xtra cost) will keep them handy.
5. If you do a lot of tubing work, ultimately you'll want some 90-degree corner clamps. The Bessey 90-degree welding corner clamps are pricey but work better than nearly any other clamp and the screws are copper plated so spatter won't stick to them.
6. Clamps. There aren't enough clamps in the world as far as I can tell. I have about 15 welding clamps and still run out on certain projects. You need all different sizes. Small work takes small clamps, big pieces take big clamps -start collecting welding clamps now. The Vise-Grip C-clamp style with swivel pads are the ones I use the most.