What miocene22 told you is correct, but I do not think those are things that must keep the simple user preoccupied.
.. any user would be able to visit the incomplete wp site by going to that directory
That is true, but only if the user KNOWS it exists..
and it might come up in google results which is undesirable during development.
I am not sure that it is a problem for the simple user, the SEO is something that concerns more professional users, and at any rate, google will update the links as soon as you move to the original location.. In my experience, it never took more than 2-3 days to update..
..Another suggestion is to develop the theme locally on your pc by using xampp...
xampp is indeed a wonderful solution (I use it myself), but I think that it is more for theme development other than whole sites (although experienced users, including myself sometimes use it so). If I understood correctly,irinakremin wants to develop a whole site, and not a theme... that means uploading images, content etc. In that case, using local xampp will involve DB modifications, which IMHO, is not so recommended for beginners.
irinakremin , look here to know how to have wordpress installed in a different directory (and thus, visible only to those who know it exist) , like miocene22 told you.