1) You can start with writing "principal" rather than "principle". By the way, this seems to be a very common error. The first paper recommended by Bruno Marin (above) makes this mistake. I would not start reading a paper whose authors cannot spell correctly what they are trying to describe!
2) Now, go to the library :-)
3) In a nutshell, PCA consists in looking at your data using a system of coordinates that maximise the variance and that are orthogonal to each other.