Everything you need to know about fitness
There are some fundamental concepts behind training with weights. If implemented and respected over time, they bring the wide majority of the results related to your fitness goal, whether it be losing fat, building muscle, or maintaining muscle mass. However, they are not principles merely related to the training side of fitness, but they also have to do with nutrition and the lifestyle you are willing to live in order to become a better version of yourself.
Nutrition counts
While lifting weights and training in general represent a crucial part when it comes to building muscle or losing fat, these cannot be achieved without an appropriate eating regimen. By appropriate I mean the right nutrition for your objective. What dictates nutrition first and foremost are calories. They are what counts in the first place.
There are 3 possibilities as for the quantity of calories to eat according to the target (or habit) you are aiming at achieving.
A calorie surplus corresponds to eating an amount of calories which is higher than your daily TDEE (Total daily energy expenditure), which is the caloric amount you would need to eat if you were to maintain your current weight. The TDEE is given by many factors which are part of your lifestyle and metabolic system (the most relevant being how many calories you burn during the day through activity, and what is your NEAT - Non-Exercise Activity Thermogenesis).
Being in a calorie surplus leads to increase weight, which may be an objective to have if you want to build muscle, in which case you would need to pair the caloric surplus with resistance training that progresses over time. The increase in weight can be slow and incremental if the surplus is small (100-200 kcal over the TDEE) or fast but with the drawback of gaining some fat if the surplus is larger.
A calorie deficit is the opposite of a calorie surplus. Mostly implemented when the goal is to lose fat, a deficit is created when the amount of calories eaten is lower than your TDEE. Here as well, the difference between the deficit and the TDEE determines the rate at which weight is lost. I write weight and not fat because if the deficit is high, the rate at which weight is lost will be faster, but with the cost of losing some muscle mass and plateauing more easily (a plateau refers to a stop in the fat loss process, which occurs at a certain point due to the TDEE of the person dieting adapting to the deficit and decreasing). So, in this case (which is the case for surplus as well), starting with the minimum possible deficit is probably the best choice, in order to have some space to work with along the way when the fat loss is increasingly slower and harder).
A calorie maintenance refers to the middle ground between a surplus and a deficit. Eating at maintenance does not alter your bodyweight, provided that you do not go through lifestyle changes when it comes to physical activity and calories burnt on a regular basis. What I mean by this is that if you eat at calorie maintenance but decrease the calories burnt at the same time on a daily basis and you begin to live a sedentary life, your TDEE will decrease over time, as your body would not need to be 'fueled' with as many calories due to the decrease in movement, which corresponds to a decrease in NEAT and calories burnt overall (hence less energy needed during the day).
The second most relevant elements of the 'nutritional pyramid' are macronutrients, i.e. the quantities of fats, carbohydrates, and protein. Macronutrients are the building blocks of every food. Calories are calculated based on the macronutrients present in the food, with fats providing 9 calories per gram, carbohydrates and protein contributing to 4 Kcal per gram each. What this implies is that if a food is composed of 20 grams of protein, 10 grams of fats, and 50 grams of carbohydrates, its caloric amount will be 370 Kcal (204 + 109 + 50*4).
While macronutrients are just components of foods and do not make much difference in theory (the amount of calories makes the difference in the body composition), eating a moderate to high amount of protein is recommended and necessary for athletes and for building muscle or even maintaining muscle mass (1.2 - 2 grams/kg of bodyweight). That is due to the amino acids that can be found in proteins, which play a fundamental role in repairing muscle fibers broken down during training sessions and building stronger, more hypertrophic ones.
In order to calculate macronutrients amounts to include in your diet, you want to, first of all, identify as best as possible your TDEE (if you cannot or do not want to go to a nutritionist, use either the Mifflin-St.Jeor equation, or any calculator online - be aware that calculators are not exact, but they can at least give you a rough estimate). However, trial and error will likely be crucial in the process of figuring out your TDEE, as online calculators can be imprecise.
The next step is to decide the macronutrients breakdown: start by calculating protein (1.2-2 g/kg of bodyweight), then move on to fats (roughly 1g/kg of bodyweight), and finally establish the amount of carbohydrates based on the remaining amount of calories.
Progressive Overload
In order to develop bigger muscles, progressive overload is needed. Provided that mechanical tension (training against resistance) is implemented and metabolic stress is exploited as well. These are the 3 mechanisms that induce muscle hypertrophy. Progressive Overload, is, however, a key aspect of improving the body composition and the muscle mass. This is because there must be an improvement over time in order for the body to break through plateau and adaptation (the body adapts to movement patterns and external resistance fairly easily). Which ultimately leads to growth and positive results.
Progressive overload can be implemented in different ways, among which we can find: increasing the resistance lifted over time; decreasing the rest time; increasing the number of repetitions with the same weight; using a slower tempo of execution; improving the form of an exercise.
Intensity of training
In weight training, intensity is defined as the percentage of 1RM used in a lift (how heavy you lift relative to your maximum). However, the concept of intensity also refers to the effort and focus put in a training session. Which is what I am writing about in this paragraph. Training hard (i.e. with a lot of effort, concentration, and mind to muscle connection) plays a relevant role in progressing and improving.
On the other hand, training hard does not mean training hard all the time no matter what. Whatever sport you do, there are periods which are meant to be dedicated to rest and deload, and fitness is no different. If you train hard always without listening to the body, you will suffer the risk of overtraining, which then has a snowball effect on your training routines, forcing you to take a break. So, thoughtfully inserting deload phases during the year, strategically and not too often, is a good practice for longevity and greater results.
Rest
In order to maximize muscle growth or fat loss (body recomposition in general), rest must not be neglected. The muscles grow while you sleep is a common sentence you hear in the fitness world.
Sleeping has an effect on performance, because the less sleep you have, the more tired you will be, the less focused you will be during training sessions, the slower your muscles and bones will recover, the worse the results.