It's MUCH easier than you seem to think. If it's a natural/whole food, the USDA has calculated the calories and other nutrients; if it's a purchased food from a major company, the company has done it and filed the info with the USDA. Either way, databases like the one here collect the information and organize it for you. Using any computer tracking system (the nutrition tracker here is as easy and convenient as any other, plus it's free), you just find the ingredient in the list and then type in the weight, and the computer will tell you the calories and many other nutrients. There are some foods that haven't been analysed, but most are in that list.
If you're using the recipe builder here, it will add the ingredients all up and give you the total calories and other nutrition info. You can get the info for the whole recipe, or you can say that it makes X servings and it will divide the total calories for you. The tracker here and many others actually organize the information into a chart that looks exactly like the nutrition label on packaged foods.
Weighing is far more accurate than using measuring cups except for liquids (and even they can be weighed.) Six ounces of green pepper will always have a specific number of calories, but that six-ounce pepper could be a cup if you slice it or half a cup if you chop it fine. If you measure it in a cup, you'll never really be 100% sure if you squeezed more of it into that cup than the scientist who figured out the calories did. Your ounce and hers will be the same.
As for how you weigh, it's going to depend on what you're making and what the ingredient is. If you're just having a bowl of cereal, or if you're measuring something like pasta before cooking it, you put your bowl or measuring cup on the scale, zero it out, and pour in the food. If you're about to chop a pepper, cut off the stem and take the seeds out, then weigh it before you chop. This will become instinct within days of when you start, and you'll probably find that it makes you a better cook because you can replicate your recipes exactly. "One medium onion" means different things to different people, but "7 ounces of onion" will make your recipe turn out the same every time.
| current weight: 132.0