Thanks for reaching out.As pointed out relaxation techniques help,that you can learn online.Further reasearch indicates cognitive behavior therapy is the best therapy to handle issues related to overthinking,negative thoughts and an array of thought related issues as well as anxiety n depression.

There are several self help strategies which you can acquire online by looking at reliable sources both from India another countries. There are simple breathing exercises and progressive muscle relaxation exercises which you can learn by watching videos and practice. You should avoid smoking or alcohol which are frequently used to cope up with anxiety. Try a healthy Lifestyle and ensure 7-8 hours of sleep. If that doesn't consult a psychiatrist or Psychologist, they can guide on specific techniques.

You can start CBT with a therapist.