Are they expecting you to actually implement the algorithm itself, or implement a system that uses it? (eg, Are you studying computer science, or mathematics?)
The .NET framework already comes with AES level implementations in the System.Security.Cryptography namespace.
If you actually need to implement Rijndeal's algorithm, (which is the basis for the current AES standard) - read up on how it works, the Wikipedia page on Advanced Encryption Standards is pretty good.
Also, I would look at how existing implementations are coded, although it's hard to find clear and concise code.
Here is a java-script implementation of AES:
http://www.hanewin.net/encrypt/aes/aes-test.htm[
^]
These are the links to the .js source code: java is close enough to C# you should be able to work it out:
http://hanewin.net/encrypt/aes/aes-enc.js
http://hanewin.net/encrypt/aes/aes-dec.js
http://hanewin.net/encrypt/aes/aes-test.js
Submit an article or tip