What should I do know to know the frequency from complex signal? Now I'm trying to digging the DFT function.
But nobody tell about the what do we need it at least to solve the dft from the complex signal- a signal with many frequencies

If I got the complex signal as the below,



then what should we have to know at the first time to do DFT?
Especially, What kind of things should I have to know to solve a signal with many frequencies? 

I've seen some parameter from example as the below when I googling.
sampling rate, N, nyquist rate, ..., sort of things.
In the practically field for engineer, if they got a that kind signal, how do they do at the first time?

as I know the below image is DFT equation.


then how do we decide the "N" and "m"?
When you say "complex" signal, I assume you mean just a signal with many frequencies, and not a "complex-valued" signal, in which the the signal values themselves are complex numbers. Let me know if I am incorrect in this assumption. (Actually, computation of DFT is the same in either case). 

I'm also assuming your signal is discrete.

The discrete Fourier Transform, $F(\omega)$, can be found by solving the following equation (where N is the number of samples your signal, $f[n]$, contains)

$$F(\omega) = \sum_{n=0}^{N-1} f[n] e^{\frac{-j 2\pi \omega n}{N}}$$

Alternatively, you could use the function fft() on MATLAB.