Re: First experiments and I already need help :(
Set up a test environment on your local pc and create your website, plain php and theme changes can be copied easily to a new place (another server for example), just db stuff needs to be implemented later, make sure to test this on your local pc before going to your live site.
When your sure the site is running correctly on your local pc and you found out a way to copy the db stuff to your fresh installation on your local pc, make a new clean install with the same settings on your live site, remember to notice members on this before doing the upgrade. Put your site behind a htacces maintenance referral, excluding your own ip, so you can still edit and test the site, but others get to see the maintenance page. If everything goes right, you’ll have your new site running in no time.
MAKE SURE TO BACKUP THE WHOLE OLD SITE BEFORE SWITCHING TO YOUR NEW ONE!
@ David: Yes there’s a plugin for wordpress that allows you to put it in maintenance mode as well, but then there would have to be a wp installed already, the above solution just refers to a html page through htaccess.