Most Western comics are read from left to right, top to bottom. See Jessica Abel's “What is a ‘Graphic Novel?'” two-page comic (Page 1 and Page 2) to gain a better understanding of this format.
However, Japanese comics (a.k.a. manga) are read from right to left, top to bottom where the front of the book has the spine on the right side instead of on the left side as it is with Western books.
Visit wikiHow's page on "4 Ways to Read Comics" for some visual aids on both Western and Japanese comics.
Keep in mind that not all comics adhere to this structure exactly. For example, sometimes a panel or a tier of panels in a Western comic are spread across two pages instead of just one page. This is known as a double-page spread. The general Western rule of reading left to right can still apply, but sometimes a spread can have images and text presented in a spiral or circular order instead of the common linear order. A reader's best judgment will need to be applied at times when reading a comic that contains an uncommon structure.
Worth a Thousand Words: Using Graphic Novels to Teach Visual and Verbal Literacy by 
Experiencing Comics: An Introduction to Reading, Discussing and Creating Comics by 
Unflattening by 
This Book Contains Graphic Language: Comics as Literature by 
Understanding Comics by 
