Hello everyone!
Actually I used the answer from another friend (as he wrote):
{vals, vects} = Eigensystem[m = {{0, 0, w}, {0, t, -w}, {w, -w, r}}, Cubics -> True];
P = Transpose@vects;
dd = Inverse@Transpose@vects.m.Transpose@vects;
By the way, it is really confused for a beginner of mathematica to get familiar with something like this:'Root[-q^2 v + s^2 v + 2 s t v + t^2 v +
4 q^2 w + (q^2 - s^2 - 2 s t - t^2 + 2 q v - 4 q w) #1 + (-2 q -
v) #1^2 + #1^3 &, 1]'