There are a lot of different platforms you can now use to self-publish your book but I can only talk about my own experience and the ones I used were CreateSpace for the paperback and KDP for the ebook.
This can seem like a nerve-racking process at first but it's actually pretty straightforward.
- Kindle Direct Publishing (KDP)
I know there are many other platforms available like Kobo, Apple or Barnes&Nobles but since I am enrolled in the KDP Select program, my book cannot be sold by any other retailers. I will probably make my novel available to those retailers when my enrollement ends but for now, I try to take advantage of the promotions offered by KDP like selling your book at a lower price for a limited time.
Although you will probably have to invest some money during your self-publishing journey, you will also find out that you are capable of doing a lot of things on your own just by doing a bit of research.