How can I make my nervus system strong?

Stress/Depression. If you feel your nervous system needs to be stronger, you are probably suffering stress symptoms which will lead to depression if it has not already. A mental health evaluation is the start.
Probably OK. Your nervous system is probably plenty strong, provided you are not damaging it with drugs, poor diet, excess stress, etc. The best thing for the nervous system is the same as all bodily systems; healthy diet, excercise, mental stimulation, interacting with other people, keeping busy.. Avoiding cigarettes or pot smoking, and using alcohol sparingly.