Accessibility/Browser Trouble for HTML CSS

I'm sure you've heard this 117 times in this forum, but I have been looking around, spending time on google, and I still cannot find an answer. If you have some time on your hands, please help me out!

SCENARIO:
- Designing a web site. Easy right?

PROBLEM:
- Internet Explorer is a very commonly used browser (unfortunate, isn't it?)
- I designed my web site using IE as a guide (I know, I know... mistake)
- How do I design other versions of my website to work in Firefox, Opera, etc.?
- When I get a domain name, do I only need one for all of my versions?
- Do I need to COMPLETELY redo my HTML/CSS to make my site work in other browsers? Or is it just some minor changes that need to be made?
- Do you have to pay for a domain name?
- ****Are there any links you can provide that specifically outline the differences between the standards for developing HTML in IE, FF, and other browsers?****
(An answer to that last question above would be a HUGE help)

Please don't reply with an analysis of my problem or my doings... I'd prefer help, guidelines, or directions instead. It's a karma thing, if you help me to get better at HTML/CSS, one day I might be helping you with Python or Java. So please answer whatever questions you can!

re: Accessibility/Browser Trouble for HTML CSS

Code to web standards. Test in one of those browsers. Will probably work everywhere. Sometimes even IE.
You need one for each site you want someone to access like example.com and example1.com. So two domain names in that case.
See above. In most cases, standard and valid markup works everywhere.
Yes.
There are no differences in standards. Everyone follows the W3C pretty good. And then there's IE. The sites I use for checking on problems:
webdevout.com
positioniseverything.net

Validate your markup, both html and css. Invalid markup is markup with errors. You cannot trust markup with errors.

Trust all the browsers to display what you wrote. If there are differences between FF, Opera and/or Safari, then you'll have to look them up.

If they display the same but IE is different, then IE is the problem and you have to look it up.

If IE displays correctly but any of the others are different, then IE is still the problem and your markup is wrong.

Never, ever trust IE to display anything correctly.

re: Accessibility/Browser Trouble for HTML CSS

Thats was a lot of answers! I really appreciate it. Thanks a bunch