To train well you need to follow basic rules
1 - ALWAYS start with LARGE MUSCLES
When you start with the big muscles (pectorals, back, legs), at the same time you also train the smaller muscles of THAT PRECISE MUSCLE GROUP.
I'll explain it a little more in detail
When you train the back muscles at the same time you also train the biceps, while when you train the Pectoral you also train the triceps and also work the shoulders a lot, while the leg muscles are connected to the abdominal.
So all you have to do is start with the opposite muscles, that is if you train your back, the muscles to train on the same day are triceps and shoulders, while when you train your legs or do them on a separate day, maybe giving us inside the abs, or work the legs by putting the chest and biceps together.
The training that I normally do is just that, and the results are optimal. try them when you enter the gym, my advice is 3 types of different exercises for legs, back, pectorals and shoulders, while for small muscles 2 exercises, all from 3 sets of 12 - 15 repetitions.
When you want to enter the gym, it is always necessary to warm up your muscles and stretch first of all, let's put a total of 20 minutes.
What I normally prefer is the treadmill or the bicycle, after which based on the large muscle groups you train you start stretching. here I leave you some illustrations
These remain poses to be used especially for the back and legs. very good is also hugging, pulling the elbows, everything must be maintained for 20 seconds, so the muscles stretch, and you can work your best