Now, this is something a lot of indie authors do for themselves. It’s not intrinsically difficult and there are plenty of programs and packages out there which can help you with the process. But you need to be aware of the rules about laying out the book as a whole (what goes in the ‘front matter’ before the main text, and in what order, and what goes in the back matter), and about laying out individual pages (first pages of chapters vs. the rest, how page numbering works). You need to understand about balancing the fonts used in the text with those used in headings. And finally you need to understand the differences between formatting print books and eBooks, and the subtle differences between different eBook formats.
If you are prepared to take the time to learn the rules, then by all means tackle this step yourself. But a slip-up at this stage can make your book scream ‘amateur.’ So if you are not completely confident, it is worth employing someone experienced so do your interior formatting for you.