There's a million horse owners out here and you'll probably get at least that many differing opinions on how to care for your horse. Pick what makes sense or doesn't sound harmful and skip the rest.
For skin conditions such as ringworm (skin fungal conditions) there are a lot of OTC products out there. You do have to apply them fairly regularly for them to work. My "go to" products for fungal stuff and for the first bath of spring, are Eqyss Products. I start with the MicroTek shampoo, followed by either the MicroTek antifungal spray or gel if there are big patches and the horse will allow me to touch him in those spots. Make sure you wear gloves, you can get ringworm and other skin stuff from the horse. After about 3 days of treatment with the spray or gel, I usually see improvement. Keep it up until it goes away. I generally wash once a week with the MicroTek shampoo and treat daily with the spray or gel. Make sure the horse can get totally dry and has a dry place to stand/lay, fungi loves moisture.
Tip for winter blanketing: Take it off at least once a week and curry and brush really well. That will help keep anything from getting started.
There is a really stubborn fungus here in OK that I've had to have one horse treated for, it took I.V. Iodine and that's a little risky, so I've become a bit of a fanatic about the grooming and dry conditions.