Every signal integrity engineer should be bilingual- able to speak in both the time domain and the frequency domain. This applies to signals and to the electrical properties of interconnects.
The basis of translating between the time domain and the frequency domain is the FFT and inverse FFT. If you have not done so, you should sit down and calculate the FFT of an ideal square wave in the time domain to see what its spectrum is in the frequency domain. After you do one by hand and see the underlying principles, its not important to do others by hand. Instead, you can use various simulation tools.
As a review, Teledyne LeCroy has put together a handy little chart with a dozen time domain waveforms and what they look like in the frequency domain, as an amplitude histogram, and as an autocorrelation. You can download this simple chart here.