"Watcha gonna do" is not a 'thing', Mohammed. It's just a non-standard way of SPELLING the very normal question "What are you going to do?". I'm sure that you've learnt "What are you going to do?" in your English lessons. But if you listen to people saying "What are you going to do?" when they're speaking quickly, you'll notice that the words run together so that it sounds like they're saying "Watcha gonna do?". Should you say "What are you going to do? as "Watcha gonna do?" Yes, why not? That's what we all do when we're speaking, especially when we're in informal situations. Saying this could make your spoken English sound more natural. Should you write "What are you going to do?" as "Watcha gonna do?" No, definitely not! You might read this in song lyrics or video captions, but we don't write it. Writing in this way looks lazy, rude and childish.

Decide what kind of native English you want to speak.(American, British) Listen to 1000s of hours of native speech. Speak 100s of hours with native speakers. Focus on saying very simple things well. Mimic. Remember that natives can understand really lousy speech, so being understood shouldn’t be your goal.