What is the best way to stop caring about someone who doesn't care about you!!

First, you must recognize that you're
attachment to another is amplified by the
insecurities you harbor. You must accept this, and then you must forgive yourself. Instead of trying to gain the approval of another, approve of yourself, and practice loving yourself with positive affirmations and positive imagination. Once you fully accept and love yourself, the approval of another will have little to no effect on your peace of mind, and your deepest
desires will attract to you magnetically.

Also, we cannot stop thinking about someone that easily, but time brings the change. Every human is unique and we cannot always expect the same thing from others in return as we do. Accept the reality that, those who doesn't care for you are not really worth for you. You deserve better in all possible ways.
Say this to yourself everyday when you wake up from bed. Good things happen tho those who do good to others, and let you wait patiently till you find someone who cares you so much than you ever do. May god help to those who help themselves in getting out of this pain.

Time is the only way to heal completely. It is impossible to automatically stop caring about someone. However distractions can help. Remember that every second you spend focusing on the person who doesn't care about you is letting them have control over
your thoughts and feelings.

By discovering yourself, when you discover your talents, you have time to focus on you and that is, how you realize you are worth more than that person that makes you stop caring for them and start taking care of yourself.

The best way is to stop focussing on them and focus on you! Take up a class, start a new hobby, reunite with friends you haven't seen in a long time. The more you do for yourself, the more you'll begin to realise what's really important to you.