Need Help... Desperately? Does anyone here do work for people?

I need help moving content around. I'm redoing my site on a domain that's password protected (I call it my development domain). When I've completed it, I plan to move it all onto my actual domain.

I'm about 1/4 done and I was told that I probably should have set it up as a subfolder or subdomain. I was also told that I "MUST root all urls though otherwise you will need to rewrite everything when you transfer it back."
(I mean when adding links and images ensure the urls are href="/wp-content......."> not target="_blank" href="">

Obviously, I'm doing it wrong. Like seriously - Who would ever know that?

Anyways. I need help figuring this out. It's driving me crazy and I'm super stressed out and want to swear. I just don't want to do anything else that's wrong because this is such a complete waste of time and more expensive fixing in the long run.