I really need help. I'm not able to be concentrate on anything, especially prayers. I always have negative thoughts about everything, even when I'm praying. And I don't only think negative, my thinking is actually bad. I believe in Allah (swt) and I know that He will always answer my prayers when I truthfully ask Him, but I'm not even able to ask Him to help me. I feel that I'm lazy to even ask Him anything. My exams are approaching and I know that I'm not making enough Doua. When I pray, my mind is always somewhere else.

I don't know how be concentrate when doing anything, and how to remove all those negative thoughts from my mind.

I would suggest u to start zikr, maybe any which you feel its easier and increase the number slowly, specially a few minutes before namaz. Inshallah it will help you.
Secondly find some mind exercises for concentration.