Generally speaking, it doesn't matter whether you use British English or American English in a story, so long as you keep it consistent:
If you use "sceptical", you will need to ensure you use British English throughout ("colour", "car park", "realise", etc).
If you use "skeptical", you will need to ensure ...
Because you're self-publishing, there is no "house" style.
Let's take a look at your example, "skeptical vs sceptical" on GoogleNgram:
You can see that the American version is much preferred.
Most of the time, it is best to use American or British English rather than Canadian or Australian because they are the most popular.
Other English ...
A pretty well-known self-publisher, BookBaby, has an option for publishing in Hebrew:
KDP has an option too, except it is only in paperback, ebooks are not available at this time:
Legally at least in the US, you'll need to be at least 18 years of age for a contract to be binding. That said, a parent or guardian may be able to accept on your behalf. However, this will likely vary on a publisher-by-publisher basis.