For a starter, just create a PDF, but with a reduced page size. Instead of setting the paper size to A4 or US-letter, set it to something more similar to the real screen sizes, 9×12 cm (3.5×4.7 in) is a popular size. Reduce margins to a minimum (1-2 mm) and the PDF should pretty readable in most ebook readers. Depending on the kind of book, this may require an extensive reworking of the layout.
The advantages of the PDF are that it's easy to create, you probably already have all the tools and skills for that, and that it should look the same in every device (just remember that there are many devices with no colour).
Then you can concentrate on creating the ePub (or mobi), which, as a first approximation, is similar to creating a web page with very few bells and whistles. If the book relies on many text boxes, fixed pagination and relative positioning of images, then a "standard" ebook is probably not the most adequate choice. If, however, it like a normal novel, with a single text flow and some interspersed images between paragraphs (or can be conveniently rendered as such), then conversion is straightforward.