Best way to learn how to cook is to get a recipe and make it. Learn from your mistakes.
That's good advice. Another is to learn what you're doing so that you can later make things up, deal with what's on hand. Starting with processed foods not only will make the food not as good or nutritious, but it's taking away from teaching you how stuff works.
You need a meat thermometer or a good finger at first. You can cut to the center of the largest part of the meat you are cooking and poke it with a finger. If it is very hot, the meat is done for sure. With a thermometer, poke it to the deepest/thickest part or piece that is least cooked and read it. The longer you cook it beyond this point, the less juicy it will be, even if you are basting it. The water leaves the cells, so even if you pour water on top of the meat, it's still going to be dry.
Pork - 160 degrees or above is done
Chicken - 180 degress or above is done
You have to cook pork and chicken to "well done", they're full of disease and aren't appealing when not cooked fully. You can eat beef raw, so cook it how you like it.
Potatoes - seriously, don't do the box stuff. You got 3 easy ways to make potatoes.
-You can boil them with or without the skins. Quarter them or half them. You bring water to a boil, put the potatoes in, and take them out in half an hour. Done. You can then mash them up if you want. You can add stuff to the mash if you want. You can mash it so there are still chunks. You can save time by using an electric mixer to mash them. You can keep mixing them until they start to fluff up and become "whipped potatoes". The stuff that people often add to potatoes are cooked onions, cooked garlic (raw garlic is almost useless), ranch dressing, milk, stuff that sounds good. Taking some garlic, chopping it up fine, cook it in a pan with a wee bit of oil (this is going to take 60 seconds) and adding it to your potatoes = garlic mashed potatoes. You can also just sprinkle in garlic powder or garlic salt to do this. Or you can just put butter in there. Usually, I keep the skin on, mix the potatoes up, and put nothing in them. I are Irish.
-Bake them. Put the oven on 400 degrees, let it warm up, throw the potatoes in. An hour later, you have baked potatoes. Unless you are using very large potatoes, then two hours. I use reds for everything, and they're never big. Some people put butter in them, some people put sour creme in them, some people cut them, throw cheese on em, and then throw them back in the oven. I uh... I eat them.
-Bake them in the microwave. The result is more like a boiled potato than a baked one. Reds, 5 minutes on full power. Big yellows, 8 minutes on full power. Then, like always with a microwave, allow them to sit. 5 minutes of sitting will finish the cooking. Mash them, or eat them like they were baked.
Microwave cooking - full power is good for baking potatoes and boiling water. Pretty much everything else you want to use a lower setting. I use between 30-60% power for just about everything. When reheating something that is milk based, add a little milk because the microwave removes liquid from whatever you cook. If it is water based like pasta, put in some water. A little squirt bottle is a cool tool for this. Just so that your pasta doesn't harden when you reheat it.
-Pasta. Find out how well you like your pasta cooked. Al dente is when the pasta is still hard and how pretty much everybody I know that isn't "weird" likes their pasta. By weird, I mean people that eat a lot of processed food or fast food. Those people love strange things. Like soggy pasta. You just cook the pasta a couple minutes longer than the shortest time suggested on the box and you have yourself soggy pasta. Make sure you always add your pasta to a rolling boil and keep the heat high. Make sure to stir the pasta every minute or two or it can stick together. Some people add salt to lower the boiling temperature, some people add oil to make the pasta not stick as much. Some people use fresh pasta because it is BETTER IN EVERY WAY but cost. For cooking fresh pasta, add to a rolling boil and only cook it for a minute or two. The dry pasta you get in stores is just fresh pasta that was allowed to dry out. It takes longer to soak water up and cook than fresh pasta which just has to cook. The best way to make pasta is to have your sauce hot and ready in a pot. That way when you drain the pasta, you just throw it right into the sauce and it sticks. Some people (a lot of restaurants) put cold water over the pasta. This makes it stop cooking in its own heat and makes it so it doesn't stick to itself. It also cools the pasta and makes sauces not stick to it as well. If you are just cooking pasta to put in the fridge, it's not a bad idea. But I never do that, so I throw it right into a sauce that is ready. A one-pot solution is to cook the pasta, strain it and leave it in the strainer. Then heat the sauce in the pan you cooked the pasta in, and throw the pasta back in when the sauce is hot. Serve from there.
BEEF! Alright, hamburger is easy stuff to work with. The higher the fat content, the easier is to cook. If the fat content is over 10%, you don't need any additional grease to cook it. You just put it right in a pan. You have 3 easy options: hamburgers, meatloaf, meatballs. For burgers, you shape your burgers with just enough pressure to get them into shape. You can add some stuff to them if you want. Sometimes I put diced onions into the mix, a lot of friends put garlic powder into them. Pepper is a good idea too. As is Worchester. If you are using a non-stick pan, don't use high heat. Use 3/4 or less. Non-stick surfaces don't work at high heat, and they are ruined. Don't use any metal tools on them either, gotta use plastic or wood. If you are using a plain steel or iron pan, just flop them down and use a nice steel spatula. Flip them only often enough to avoid burning. Don't press down on them, it shoots all the juices out. You want to heat the juices, not remove them. If you're in a big hurry, pressure will cook the burgers really fast. For meatballs and meatloaf, it's easy. There are tons of recipes online. I always have eggs, worchester, and bread crumbs on hand, so it comes down to: do I have red pasta sauce on hand? No? Meatloaf! Yes? Meatballs! I put eggs and bread crumbs in both, and worchester in my meatloaf. I also put a bunch of veggies in meatloaf, julienned up really small. Da red sauce goes in da meatballs. Sometimes I add some spice to the meatballs. I'm not above just tapping some tobasco sauce into the mix. Oh, and I put some parmesean or romano into the meatballs, not a lot, but enough to give some taste.
Buying beef - buy the stuff with the thin plastic that touches the meat. That means there's a good chance they ground and packaged it there. The stuff that has a thicker plastic is sealed without oxygen inside, so the meat doesn't turn grey from oxidation. The meat can go bad, but won't LOOK it. Also, they very often add tons of red food coloring in these packs. You'll have bright red hands after working with it. And if you didn't know, dyes are pretty much the most unhealthy thing that is still in food. Dyes and preservatives, I guess.
I'm sure somebody else will tell you about cooking chicken, I'm gonna stop typing.
Make sure your knives are sharp. If you can see the edge of the blade, it isn't sharp. You can hurt yourself with a dull knife.