Find the frequency response if i have the magnitude response?
The frequency response of a system can be represented in polar format, in which the magnitude and phase response are considered separately:

$$
H(\omega) = |H(\omega)| \angle H(\omega)
$$

With this representation, it should be clear that the magnitude response alone is not sufficient to characterize the full frequency response of a system; you have to know (or assume) what its phase response is also.