How to build my muscles, now i'm 4month away from 40yrs?

Weight training: The best way to increase muscle strength is with resistance exercise, 3 times per week that gradually increases the load ( or the amount) of work the muscle needs to do. The resistance can be weights ( like barbells, machines ) or your body weight ( push ups , other calisthenics.) for the details discuss with local doctor, fitness trainer or coach. Consult with dr before start of new program.