The cord of one of my testicle suddenly got hard but when i touched it , it got soft and normal in seconds, i do not experience any pain in my body nor any nausea or fever im going through puberty ? im really scared , parents say its normal at my age?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. What you described does not suggest torsion. It would be prudent to see a doctor if you get more than mild pain. Wish you good health!

If the testicle is not higher up than the other and not painful then there is nothing to worry about. Wear a support underwear.