~Luke

Remove float: left; from .column
Add display:inline-block; to .twelve and .polaroid-one, .polaroid-two, .polaroid-three
Change the width on .column-content to width: 100%;

If you update the live example, I can take a look.
 
Whenever I'm in this situation, I always put an outline or border on the elements to work out what's going wrong. If something is too wide, you will see it instantly.
 
You could do something like:
* { outline: 1px solid blue; }

Nivo Slider is free, you only need to pay for the Wordpress plugin.

Personally I use Cycle 2 and extend it as I need.
http://jquery.malsup.com/cycle2/

Although Owl has caught my attention.
http://www.owlgraphic.com/owlcarousel/

You can find a lot more at:
http://www.unheap.com/section/media/sliders-carousels

Ajax and jQuery are both JavaScript, the former being a way of dynamically loading content, the latter being a framework to make coding JavaScript faster and easier, jQuery also makes Ajax much easier to code (without cross browser ajax can be a nightmare).
 
If you have learnt html5 in any real depth then you should already know that xhtml is what html was prior to the release of html5 and there is no point in learning something that has been replaced.
 
PHP is a great thing to learn but it does sound like you're headed more down the front end developer route, so while it is useful to know some of the basics I wouldn't go much further if this is your intention. If you do wish to become employed as a developer then absolutely learn php, along with one of the many CMS systems that are built on it like WordPress.
 
If you want to get employed as a front-end dev then learn JavaScript. Once you have a good solid grasp of this then move onto the frameworks, jQuery being the most widely used and you should certainly study that. JavaScript takes a long time to get competent at and an even longer time to get good at it, but being able to build custom plugins rather than relying on pre-existing ones will make you much more employable.
 
Responsive development comes under html5 and css so continue to study those in greater depth also. Being able to code a responsive site now will be required at almost all agencies.

Hi
 
Firstly, "it's never too late", more so at 26, you're still a baby
 
Secondly, you can purchase Photoshop and Dreamweaver but I tend to find GIMP as a good, free alternative to Photoshop. Dreamweaver, nah, use Notepad++ - or similar.
 
Thirdly, simply use Google, there are a wealth of websites out there. Smashing Magazine, W3 Schools etc, there are far to many to mention.
 
Finally, welcome back to the world of website design!!