I think before starting of these thoughts,there must be some cause which work from behind to cause u sadness.
Try to devide ur time with frindes, family,work,study ,passion.
Watch some movie or any video which motivate you.
Try ur hobbies this time to fight with these thoughts like drawing,singing,dancing,playing etc.
Dont loose hope,what are u good at ,concentrate on thaose things.
Health Tips
for overall health
1.sleep 8 hrs at night.
2.avoid too much social media.
3.avoid too much use of mobile pc laptop.
4.avoid mobile pc 1hr before sleep.
5.before sleep may hear good music,or read a good book or can watch a good movie.
6.workout daily or regular morning walk or join gym.
7.Share ur thoughts with ur parents or good friends.
8.Concentrate on ur work,study,passion, hobby.
9.Maintain proper diet.
10.Take proper protein supplement and some time multivitamines.
for more help consult psychologist or psychiatrist.