If you're doing artwork traditionally(via paper, pencil, and inks) you can buy Canson comic boards from the local(or nearby art store) that have the dimensions marked. If you're drawing digitally(via art program) find a comic page template to download; convert it to a PNG file, in which you can save it somewhere & use it by adding layers on top of it to do the artwork on.
You may already know some of that, but I use Ka-Blam to print my books. Once your page is done, they have PNG templates that you can download and place your finished piece on top; if it fits the template then your page is complete- if not, you may have to adjust your page to fit the perimeters of their template. Ka-Blam will take your pages if they are RGB(I believe they do the conversion themselves). The only thing you need to is make sure that your finished pages are PRINT READY(check for spelling/errors, make sure the page is in the required format that they use, and that it fits in the perimeters of the template).
You can convert files to CMYK through Photoshop(if needed). Go to Image>Mode>CMYK.