There are many reasons for low stamina in men. You must consult your doctor for improvement. Generally this problem occurs to people suffering from diabetes, hypertension, psychological disorder, etc.
Now a days masterbation is common problem. If you have a habit of it and unable to leave it, you must consult to Psychiatrists. Once a while is not problem as body itself generates sperm. But daily/regular habit is very bad for your health.
You can improve your stamina using following tips:
Exercise - Regular exercise is most important to boost stamina. It helps for blood circulation throughout body. It repairs all weak cells and tissue. Push up , aerobics exercise like walking or running are considered as good exercise for this purpose.
Yoga - Various yoga is suggested for it as breadthing exercises like Anuloma-Viloma.
You can follow following link for it
Home item - Various home used item works effectively like garlic and ginger. You can take 2 to 3 garlic peels in empty stomach in morning. You can take around 4 gms of ginger per day directly or with any supplement
Medicine - There are many medicines available in market for it. Mostly has fake promises. Some ayurvedic medicine works effectivey in it are extacy from Nupal , Ashwagandha and Omega-3 fatty acid. Please consult to your doctor before taking any medicine.