Can you post an example question or the chapter you are doing right now in class? This question is quite general and a vague answer won't be of much help.
Assuming you have a simple network made only from resistors, capacitors and inductors you can calculate their complex impedances by the formulas:
\(Z_R=R\\
Z_C=\frac1{j \omega C}\\
Z_L=j \omega L\)
where j is the complex unit and ω is the frequency of the voltage applied on each element (coming from the voltage source).
For a more simplified analysis on extreme cases, you can substitute capacitors with open circuits and the inductors with short circuits, when the network is operating on very low frequencies. Resistors stay as they are.
For very high frequencies capacitors are substituted with short circuits and the inductors with open circuits.
I suggest you read the AC Volume of the All About Circuits E-Book. A link is on the top of the page.