You are in the right path. Proceed with your learning with html, css and then go for php and mysql, then the whole of the internet world is yours!
You can just play around with these languages to create powerful websites that can facilitate features like carts, login (can be effected through php 'sessions') and almost what ever you want.
Besides these i would suggest you learning some JavaScript(you dont need to master that. just a few basics will do for form validations, etc)
And if you really have much time, then you can learn AJAX. AJAX is just to enhance the browsing experience, like when you place your mouse pointer on name to the left here, it displays my score, best answers etc. This is one example for ajax.
If ajax were not to be employed in this scenario, then it would have been like you should click the my name here and it ll take you to a new page and display the score, etc only there.
Thus ajax can be like some sort of enhancement on your webpage's design and is not really needed for every website to use ajax.
So, for now concentrate on learning HTML with CSS. Its the ultimate frame where all the other languages work on.
You might also wish to refer some good tutorials out in the net. Here are the links to some good tutorials :
http://www.w3schools.com/htmL/
http://www.2kweb.net/html-tutorial/
http://www.w3.org/Style/Examples/011/firstcss
http://www.pageresource.com/jscript/index2.htm
Hope this helps. Happy learning! :)