You should choose a machine over a compound exercise if you're just starting out because you are less likely to perform them incorrectly and injure yourself. Most fitness articles will mention things like a bench press, back squat, and a deadlift, but I will argue that you're better off choosing a different set of exercises, especially if you're significantly overweight.
Mental tax
If you have never been to the gym before then it can be an overwhelming place to set foot in. You don't want to be worrying about whether the exercises you're doing are actually going to make a meaningful difference. Adding complex form requirements won't make it easier for you to succeed. You should keep your program as simple as possible until you've mastered the parts that matter. Machine exercises simplify this process by offering clear guidance through their design.
Progressive overload
The one idea you need to master is that of progressive overload. If you're not steadily pushing more volume week after week you're wasting your time. Your progress should look roughly like this:
Week 1: 8x60kg
Week 2: 8x65kg
Week 3: 8x70kg
Or like this:
Week 1: 8x60kg
Week 2: 10x60kg
Week 3: 12x60kg
Notice the patterns.
There are different ways to achieve the same thing. That work is taxing enough as it is. Complicating the work by adding the complexity required to perform compound movements means risking injury and slowing your progress.
Minimising risk
You should choose machine exercises over free weight exercises because they minimise risk of injury. These machines usually force you to perform the exercise along a specific path. They usually require minimal instruction, reducing the likelihood of improper form or technique that could lead to injuries.
That's not the case for dumbbell and barbell exercises. You can injure yourself very easily and they're difficult to master. If you have access to a good coach I still highly recommend learning them, but if you're a total beginner just stick to the machines. The benefits of free weights provide over machines are marginal.
Exercise selection
What exercises should you be doing then? For my clients who are a bit overweight I do the following:
- Leg Press: Good for lower body strength and easy to execute.
- Leg Curl: Targets the part of the leg the Leg Press doesn't (hamstrings) to round out leg development.
- Seated Shoulder Press: Focuses on "above the head" pressing strength.
- Seated Row: Improves pulling strength.
- Lat Pulldown: Great for upper body strength and the first step to doing a pull-up.
- Chest Press: Good for pushing strength.
For clients with large bellies, substituting leg extensions for the leg press can provide a comfortable alternative. These exercises form a balanced routine that covers major muscle groups without overcomplicating your workout.
You're good for about 6 to 12 months before you need to think about splitting your workouts into different groups.
If you have specific goals you wish to achieve, for example a bigger butt, then you'll need to program for that result. The above program won't get you a bigger rounder behind. It's meant solely as a starting point, and it assumes you're overweight, not obese.
Cardio Integration
Cardio should complement your strength training. For beginners, start with a moderate-intensity workout on a treadmill at an incline for about 10-20 minutes and try to keep your heart rate in Zone 2.
Final thoughts
While compound exercises are valuable for advanced trainees I don't think they're a good idea for beginners without expert supervision. Focusing on machine-based exercises reduces injury risk and simplifies the process of learning the fundamentals of strength training. There is no such thing as the 'perfect program'. Each individual trainee is different. Hiring a coach can be an excellent next step in learning what gets you closer to your goals.