1. Prioritize Compound Movement/Lift Exercises over isolation exercises. Compound lifts are exercises that train multiple muscle groups simultaneously. In contrast, isolation exercises only train a single muscle group at a time. Some prime examples of Compound Exercises include: Deadlift, Squat, Bench Press, Pull-Up, Overhead (Military) Press and the Barbell Row.
In regards to how many reps (repetitions) you should do, studies have shown that 8-12 reps produce the most gains in muscle size and should be around 80% of your one-rep max. One-rep max is simply the maximum amount of weight you can lift once.
Once 8-12 reps becomes easy, you'll need to add more weight over time, otherwise you will hit a wall/plateau in muscle growth. Aim to train heavy, but without compromising form. No one wants to get injured and be sidelined for weeks.
2. Take time off to recover because muscles grow between workouts, not during. Take at least 1 to 2 days off per week to all your muscles to fully recover. For example, a couple of popular training routines include:
a.) push, pull, legs, rest
b.) legs, chest, back, rest
Overtraining will actually stunt your muscle growth plus increase the risk of injury, so do make sure to give your body a break even though you are excited to get back to the gym.
For weightlifting, training between 45 to 60 minutes per session should be more than enough.
3. Get plenty of sleep because it's when you're alseep that your muscles are recovering and your body is growing. Ideally you want to get 8 to 10 hours of sleep, but for most of us, that's extremely difficult to do.
Whether you're a husband and/or father that needs to spend time with them, or work a job that requires more than normal hours, a vast majority of middle aged men just can't be in bed for that long. Aim for at least 6 hours of actual sleep. That means, put the phone down, schedule to go to bed and wake up at the same time, and sleep in a completely dark, quiet, cool room.
4. Eat enough and eat right. If you're looking to build muscle, you need to eat a surplus of calories. If you're looking to lose weight, then you need to eat in a deficit. Use a popular app such as MyFitness to find out how many calories you should be taking in, and then go from there.
Clean up that diet, eat enough protein, a good amount of vegetables, and yes do include carbohydrates and fats. Carbs are actually important for muscle building because they're protein sparing, which means the body looks to glycogen for energy instead of breaking down muscle tissue for energy. Consuming carbs post-workout can prevent muscle loss and help repair muscles.