Gauge of wire used to build with is highly dependent on the target ohms and the build deck of the rba being built.
When first starting to build, found 28 gauge to be best, was building at 1-2 ohms at the time. As my power went up, skills developed, me target ohms dropped. Now I build at 0.4-0.8 ohm, with a few in the range of 0.15-0.25 ohms. Will choose either 22, 24 or 26 gauge, depending on gear and target ohms.
It's best to get a sample of various gauges, to find what works best for your vape. Feel tension wrapped micro coils to be one of the best methods to becoming an competent, consistent builder.
Spaced coils are generally, free wrapped, or hand wrapped, around the wick, then mounted. Micro's are generally wrapped around a mandrel, drill bit, blunt tip syringe needle, basically anything round. Once mounted, pulsed, squeezed and set, the coil is then wicked juiced and vaped.
Any sterile or organic cotton is adequate to get one started in the wicking arena. But, just like wire, there are many different fibers to choose from too. Heck, besides the multitude of cotton fibers available, one can choose degummed Hemp, Ramie Top, Rayon, heck, there's even a seaweed fiber blend that looks very promising.
Main thing is just to get started. Get used to building, learn the basics, and go from there.
Personally, my purrefrance is tension wrapped coils, regardless of diameter, and Ramie fiber. Also use hemp fiber for some setups, like the nutty/woody taste it imparts on some flavors.
Vape long and Prosper.!!!