It used to be that in order to change the format of chapter and section headings, you needed to have some understanding of the internal workings of classes such as report or book. Modern classes, such as memoir and the KOMA-Script classes, provide a much easier interface. However, I recommend that you first write your thesis, and then worry about changing the document style. The ability to separate content from style is one of the advantages of using LaTeX over a word processor. Remember that writing your thesis is more important than the layout. Whilst it may be that your school or department insists on a certain style, it should not take precedence over the actual task of writing.
This book is also available as A4 PDF or 12.8cm x 9.6cm PDF or paperback (ISBN 978-1-909440-02-9).