Here is the recipe for one gallon of yogurt.
Use milk that you like. There is a good chance that you will like the yogurt that you make if you like the milk from which you make it.
Add the heavy cream or half & half if you like a thicker, creamier yogurt. If you choose to add either of these, make sure that there are no additives in the cream or half & half. Many companies, even organic processors, will put stabilizers in these fatty milk products to prevent them from separating.
Check the ingredients on the yogurt you are using as starter. Be sure it contains live cultures. Read all the way to the end of the list of ingredients and beware of any additives or stabilizers like pectin. When I have used yogurts with additives as a starter, the results have been slimy. Yuck. You can also buy freeze dried yogurt starter. That is a more expensive route to go, but the results are very predictably smooth and creamy.
Try to avoid spending lots of money on equipment for your yogurt production until you are sure that you like doing it. Many people have much of what they need for this project, even if it doesn't exactly fit into the above list.
In Wild Fermentation, Sandor Katz gives great descriptions for determining temperature by feel in yogurt making. If you want to avoid getting a thermometer, you can learn to use your finger as a gauge instead.
Everyone needs something in which to incubate the yogurt. I use a plastic cooler with towels thrown over it. Some people can use their ovens. Some people use heating pads and blankets or towels. Look at what you have and experiment with your incubation on small batches until you find a method that works for you.
In all my fermentation projects, I try to use vessels and equipment that are easy to clean. Remember, fermentation is about microbe management. Stainless steel and glass seem to be so much more reliably clean than plastic and wood. Use what you have and feel comfortable using.
The canning funnel is not necessary, but makes getting the hot milk into the canning jar so much easier. You can find one at a hardware or farm store for about two dollars.
Thick, creamy yogurt that has been cooled