For databases, you need MySQL (PDO specifically) along with most typically PHP.
Not sure what you mean when you ask what if you want to build apps that use those databases.
Most well-known web hosts should be able to handle any language you mentioned (again, I don't know about Ruby so not sure) but email support if you want to know for sure. I've used Dreamhost and I know they handle html, css, js, jquery, php and mysql databases just fine.
Most of the rest of your questions are best answered by someone else. I'm pretty new to all of this but I've learned all that I mentioned and more in a year's time.
Start with codecademy to get your feet wet. Learn all the basics there and start building websites now. It's intimidating but I taught my 5 year old son how to make a website in about 10 minutes. Simply <html>My website!</html>. It's a bit more than that but that in and of itself would work.
Check out the page source of websites. It's fantastic learning information.
Download Sublime text editor, MAMP if you use a Mac or WinAMP if you use Windows.
That's about all I got in me; I may add more later. But the earlier you start building the better.