The answer to your question depends on how you plan to publish your book.

Self-publishing: You have two choices: do the layout yourself, or hire a graphic designer. Either way, you and the designer will need to understand book layout conventions. That actually has less to do with computers and software, and more to do with knowledge of dozens of conventions like what subheads should look like, the hierarchy of subheads, widows and orphans, hyphenation, and so on. Book layout also takes a good artistic eye for size, position, and balance. If you do this work yourself without prior experience, you're in for a pretty big learning curve. If you hire a layout artist/compositor who has experience, you still need to know layout conventions so you can check the artist's work. I've done quite a few self-published books for clients. Average number of layout errors per book when done by someone other than a publisher: 200 to 600. Average number of proofs needed to get everything right: 6. That means someone has to check the entire book 6 times to find the errors and get the compositor to fix them. You have to keep a chart of the errors and track whether they've been corrected.

Traditional publishing: For traditional publishing you make NO design decisions. You submit a very plain manuscript that uses NO special styles. The entire manuscript will be done in "normal" style. If you use special styles, the publisher's compositor will begin by stripping all of those out of the manuscript. However, you do need to know what the definition of "plain" formatting is. Here it is:

--12-point Times New Roman throughout. Even for titles.
--Double space absolutely everything, even footnotes and references
--Indent paragraphs .5"
--Don't leave an extra line space between paragraphs
--Left justified
--No end-of-line hyphenation
--Use a clear hierarchy of subheads. Example:

Level One Subhead
Level Two Subhead
.....Level three subhead. Indented. Continue typing on same line.

The subheads are the only formatting you do, but don't use the style menu for the subheads. Do them manually, as in control B for bold. Again, the publisher will strip out the styles if you use the style menu. Microsoft Word's styles have nothing to do with the publisher's choice of style; they interfere with the publisher's work.

What exactly is a book designer and/or compositor? Well, a ghost writer does not have this skill, so skip that. A book designer makes decisions about the appearance of the book. The designer should make you a sample of the first two pages of a chapter for you to approve. The compositor (formerly known as a typesetter) applies the designer's decisions throughout the book. Some people have the skill to do both jobs. You will also need a cover designer.

A word of warning: don't spend any money on book composition until you've had your book professionally edited, unless this is a book for your family and friends and you don't care about the quality. Once the composition has been done, you will be charged for changes. A good publisher will do the editing for you.

In sum, here's the whole process for a book you care about:

Write your book
Get a developmental editor to give you feedback on the content and organization
Revise
Get the book copyedited to correct all mechanical errors (grammar, spelling, etc.)
Get the book proofread (a separate step from copyediting). A good editor can do all three kinds of editing.
Get the manuscript formatted for a traditional publisher, or do this yourself
Self-publishing: interior design, composition, cover design
Check the proofs, possibly multiple times

The only reason for you to do your own design is pretty much for the fun of it, or to give your designer an idea of what you're looking for. I've never looked for a book template online, but I believe there are some.

Despite everything I've said above, publishers may ask you to format your manuscript in a certain way when you submit it. Check for "author guidelines" at the publisher's website.