Hi Debbie,
If you are just going to have an informational site then an HTML editor is probably all you need. Or you can code from scratch and use Notepad and put those classes to use right?
CMS stands for content management system and it's a program that manages dynamic sites like blogs or sites that have a lot of user interactivity. WordPress would be an example of a CMS. No need to learn that though if you don't want.
As far as an "under construction page", I've never really understood the need for those. If you think about it, unless you advertise your site, people aren't going to know how to find it when it's new. So why not build it up until you have enough to launch with and don't worry about an under construction page.
So I guess your next step is to decide how you are going to build your site. Are you going to use software like Dreamweaver, Nvu, etc. or are you going to code from scratch?