Layout is something that can take a while to learn. Unfortunately there really is no how to do panel layout manual, just a how to NOT do a panel layout manual. Essentially design (which is the essence of panel and page design) is something that is learned through experience and trial and error, not a lesson book, since there are literally millions upon millions of ways of lay out a comic page, and each can have an appropriate use for some type of story. The trick of it all is training your eye to see the five to ten possible options and choosing the option that will work for you, and for the images you have chosen for that page and the story you're telling.
Another option is to ignore layout completely and do a set size and format. Many webcomics such as Penny Arcade, Megatokyo (for the first couple years), PVP and others use a set format, or even a set number of panels per page to make thier comics.
No one really worth listening to, unless you plan on EXACTLY copying their drawing and layout style, is going to tell you that THIS is THE WAY to lay out/draw/color your comic (I left out lettering because there are some pretty definate guidelines to that floating around out there). They may even provide a how to as to how THEY put a comic together, but you are bound to be missing something from thier process, since everyone has their own little quirks in comic construction. But thats not going to be how you put a comic together in the end. You are going to put it together YOUR WAY.
I do recommend the book by Scott McCloud called Making Comics. It covers the concept, philosophy and process behind writing a comic. While it does not tell you HOW to draw a comic. (As I said any good teacher will tell you to just practice and practice your charachters and elements so you can represent them correctly throughout a story. There is no magical way to force yourself to suddenly know how to draw charachters consistantly, just hard work. Its like memorizing songs on an instrument... I estimate that it takes me about 2-300 playthroughs before I know a song from memory. I would think that representing a charachter, or at least a coherent drawing style would take AT LEAST that much work, if not more.) It WILL tell you how the parts of a comic work to communicate a message to the reader so you can understand how to use them to make an effective comic.
Another book is his Understanding Comics that explains how comics work in the mind and why they only work in certain ways, as opposed to just words or just pictures. Will Eisner also wrote a book called Comics and Sequential Art that may be helpful, though I have not read it yet.
One thing to look at though:
http://www.tokyopop.com/aboutus/mechspec5.pdf
This is the mechanical specifications and suggestions for TP's rising stars of manga competition.
