fft, Periodogram, PSD,digital signal processing

Hi.. I want to know about the coding part of the fft, peroidogram and PSD. Because i want to compare frequency present in any signal by all of this whether its coming right or not? It means if i will write only PSD of any signal then it gives me frequency but i don't know what is scaling for that. I typed open fft but i don't understand how to interpret that. Can anyone help me to figure out this?