How can I stop insomnia?

Depends. Depends on if the insomnia is due to stress, psychiatric disorder, a primary sleep disorder, another medical condition, medications or poor sleep hygiene. The best treatment is addressing the reasons for the insomnia. Chronic insomnia is best treated with a behavioral program called cbt. Medications are usually indicated for short term use.
Many ways. Need to define the cause if you can, then treat the problem. For ideopathic insomnia or shift work, you may need a hypnotic medicine. Otherwise, insomnia can be from anxiety/depression, urinary issues and bph, arrhythmias, hormonal, restless legs, sleep apnea, etc - can try a simple improvement of sleep hygiene, avoid stimulants & cut stress. Can try melatonin, cammomile or valarian root.