Sigil is an eBook editor that you can use to convert your manuscript to a fully validated eBook.
You can use online tools that will create an eBook for you, but often the quality of the file is lacking. Whether the application leaves excess code from the word processor or files to do simple things like linking titles back to the table of contents. Sigil is the answer because you can fine tune your eBook, even if it is just some minor corrections after an online tool did most of the work.