Hi Jawaid,

As you are just stepping into High School, you still have plenty of time. If you work wise, you can manage to get very good scores. It is mostly upto you and your preparation, how you would perform in the competitive exams. 

Some of the best recognised coaching institutes are,

1.Aakash

2.Allen

3.Resonance 

I don't think any of these institutes give training for both NEET and JEE-Mains. Most institutes urge you to concentrate on any one and not both. But if you insist on preparing for both, you can preapare on your own. 

Time Management is very important. Get materials and study on your own. Do not strain a lot. Nothing is impossible. 

All The Best.