Feta cheese is a very easy cheese to get started with. A lot of folks will make a simple acid cheese such as Pot Cheese or Farmers Cheese, but I did not like the taste and for marginally more effort you can make a delicious, mild feta.
First, you'll need a feta cheese culture, rennet, thermometer, and cheese cloth. These affiliate links will take you to Amazon and the products I use. You'll also need a large stainless steel pot with a lid, a long knife, salt without added iodine, and a slotted spoon. You may want a large pot big enough to fit your first large pot in, as well, or you can use your kitchen sink.
Fill your large pot with 2 gallons of milk. Immerse the pot in hot water - either by placing it in the kitchen sink filled with warm water, or by placing it inside an extra large pot filled with hot water on the stove top. I prefer the stove top method as it allows me to easily reheat the water as needed. Put your thermometer in and wait until the milk reaches 86 degrees Fahrenheit. This is about as interesting as watching paint dry, so bring along a book or other small task to keep your occupied. All temperature changes must be made slowly.
Once your milk is up to temp, stir in 1/8 teaspoon of the feta culture. Allow it five minutes floating along the surface to rehydrate, then stir it in thoroughly with your slotted spoon. When you stir things in cheese making, you do so in an up-and-down motion, not a side-to-side motion. Cover the pot and leave it sitting in the warm water for an hour, the culture will use this time to "ripen".
Now mix your 1/2 teaspoon of rennet into 1/2 cup of cool water. I always use liquid rennet, because I find it gives the best results. It is stored in the fridge but you need very little, so I get it in small quantities. I've been using the same 4oz bottle of rennet for a year now, and it's still half full. Stir the cool rennet water into the ripened milk, stirring for at least 30 seconds to be sure it is fully incorporated. Put the lid back on and let it sit for an hour.
After an hour a big disk of cheese curd will be floating in your pot. The yellow colored liquid it's floating in is your whey. Cut the curds into squares with your long knife. We want them to be about 1/2" each, and use the slotted spoon to cut the curds sideways. Now, if your pot is in a sink of warm water, add more hot water. If your pot is inside another pot of water on the stove top, turn your stove to Low. We are going to begin gradually heating the curds. We want to bring the temperature back up for 86 degrees Fahrenheit and hold it there for 15 minutes. During this time, stir the curds every five minutes so they don't stick together.
Once you've cooked the curds a bit for 15 minutes, drain away the whey using a cheese cloth. (You can save the whey for other uses, but I don't always.) I tie the cheese cloth into a sack and hang it from my kitchen faucet, so it continues to drain into the sink. Drain for two hours or up to six hours, then remove the lump of cheese from the cloth and slice into slabs. You want the slabs to be 1.5" to 2.5" thick.
Rub each side of each slab with the salt. You want to use salt without iodine or other additives - I use canning salt, or kosher pickling salt, or you can by fancy "cheese salt". Salt is salt. Lay the salted slabs on a plate and leave them uncovered at room temperature - I just leave the plate on my counter. The cheese slabs will sweat whey, so check back periodically to drain off the puddle of whey. After a few hours, salt the slabs of cheese again and turn them over, and leave them to sweat. I flip and re-salt two to three times in a 24 hour period. After that, put the cheese in the fridge to age for 5 to 7 days and enjoy! Usually this is also when I will bag it up for the freezer if freezing it. I get about 2.5 to 3 pounds of cheese out of 2 gallons of milk, so that's a lot of feta at once! Plus, freezing makes it easier to crumble up for salads.