Problem with domain and wordpress

Ok, hang in there for a second and let me explain my situation with WordPress and domain problem.

First, I created a new wordpress site on my existing website (Thedallaspress.com ‘no it’s not actually a popular newspaper website that some of you know’) I created a folder for my new website and name it “/SongThanOnline” inside my existing website and continue to edit the new website that I’m about to build for my client.

After finishing finalizing the website ‘thedallaspress.com/SongThanOnline’, I then Link my client’s domain to my server so that will became songthanonline.com without the thedallaspress.com/SongThanOnline. I want every including the article and pages to transfer songthanonline.com.

So, what I’m trying to get is that when I post a new article it always goes to ‘thedallaspress.com/SongThanOnline’ and not songthanonline.com. Plus, when I use the new domain, I can’t login to the website. Is this something to do with the permalink? How do I make the full transition so that my client never see another website before his in the address bar? It’s hard to explain, but I figure you guys have this deal with this problem before. Thanks in advance.