For a paperback you upload a print-ready PDF at your chosen trim size with the right margins, plus a separate cover PDF. For Kindle you upload an EPUB or Word file. Choose the trim size first, because it sets the page count, margins and spine width.
Paperback interior
Pick a trim size (6 × 9 in is common for fiction and non-fiction), set inside margins that grow with page count, embed all fonts, and start chapters on a new page.
Use bleed only if images run to the edge of the page.
Kindle ebook
Use real headings for chapters so the table of contents works, and avoid fixed page layout tricks: readers change the font size.
Check before upload
KDP's previewer flags text outside the margins and missing fonts. Order a proof copy before you publish the paperback.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.