If you sit a pan directly on the hot coals, your food will likely burn. If you hang your pan too far above the coals, it will not cook. If you try to cook over a fire instead of a bed of coals, you will be frustrated.
So what you need to work toward is a bed of hot coals, and a pan that is not too close nor too far away from the coals.
HOW close? Well, that depends on what you are cooking, and most of that knowledge will come with experience, however you can logically expect the coals to be hotter when you are closer to them :)
Obviously you will have to replenish your hot coals from time to time as you are cooking. So in your fireplace, choose a spot that is comfortable for you to reach to cook in, and another place to the side or rear of the fireplace to keep a fire going. I cook all along the front and one side of a fireplace, and allow the fire to burn in the left rear portion of the box. Whenever the coals burn out or loose some of their heat, scoop hot coals from the fire area to the cooking area. Some of the spent coals can be removed during cooking, but I usually wait 'until the cooking is over to do that.
There are many, many items that you could purchase to use for cooking in your fireplace, but here are my favorites:
Spyders - These are three or four legged trivet-like things that hold your pans above the coals. They have a ring for the pan to sit in and an open bottom. They can be purchased in varying heights so that you can cook close to the coals or several inches away from them. Three different heights would be ideal, but two would do. The really tall ones are great for keeping food warm.
If you equip your fireplace with a trammel or hanging arm, you can buy all kinds of do-dads to hang on it and hang your pots from. I like the one that adjusts from short to long so you can adjust how quickly your food is cooking without moving the pot off the fire. This is especially handy if your fireplace is small and you are cramped for room in there.
Utensils - My very longest ones are 18". Also consider getting cast iron utensils instead of stainless or wooden ones. They just last longer. You need a spoon, a slotted spoon, a fork or three of various sizes, two spatulas, one short and one long, and that's all that is really essential. As you cook more and more you will find that there are other utensils that you would like to have. Choose very sturdy ones, for you will find that you use them for lifting Dutch Oven lids, pots and pans and other heavy items out of the fire. I finally got a utensil that is nothing more than a big hook to do just that!
You might want a split to roast meat on, but I bind the meat up with cotton thread, season it and hang over slow coals for about 6-8 hours to roast. Works well, if you can stand the aroma for that long! Another good way to roast meat like venison steaks, is to skewer the meat onto a large fork and prop the fork up in front of the fire, turning frequently 'until the meat is done.
There are reflector ovens made for the fireplace and they are really great.....once you learn how to use them properly, and that takes practice. They can be used to cook meats, breads, cakes, cookies, or casseroles. They are relatively slow cooking, but they do the job very well, as soon as you learn how to keep the coals at an even temperature and how to pull the oven back from the fire when it becomes too hot.
There are also Dutch Ovens. I recommend one with a lid that has a lip on it so that you can put hot coals on top of it without them sliding off. The coals on the top of the lid helps the food to cook from both the top and bottom of the pan, much the way a conventional oven does. This is the best way to bake in the fireplace, besides the reflector oven. You want Dutch Ovens that have LEGS. You will need at a minimum of three Dutch Ovens to cook a large meal. They can be used to cook cakes, cornbreads, puddings, soups, stews, roasts, on and on.....The trick is to keep the pan moving every ____ minutes. You will fill in the blank as you are more experienced, but I find that I like to move my Dutch Oven around and reposition it with new coals every 5-15 minutes. It's very easy to burn a cake in a Dutch Oven...How do I know? Oh....never mind.
Other Pots and Pans - Well, just get cast iron and make sure that they all have LEGS on them! You want the coals to be able to get up under the pans to cook the food, this way you don't have to sit the pan ON the coals and risk burning. Make sure they have handles, Dutch Ovens are my favorite.
You will need a safe place to sit hot pans coming off the fire, lots of dish towels and all of the usual fireplace accoutrements like a shovel, ash bucket, bucket of water for emergencies, poker.
You will need a large pan or tray to place your utensils on while you are using them.
One last thing I have learned about cooking over a fire. When I am pushed for time and I have hungry people to cook for I have to use higher heat and therefore more grease in my cooking. However, if you are not pressed for time and you can relax a bit, you can cook with lower temperature coals and use less grease. This may not seem important now, but as you cook on the fire more and more you will catch yourself adding more grease to whatever you are cooking because the temp. is too high.
You are going to have to grease the pans a lot more than you are probably used to doing anyway with conventional cooking, especially considering our low fat ways these days. But as you become more experienced, you can cut back on the grease considerably.