First you need to pick a framework, unless you plan on implementing one yourself. I used to like CodeIgniter when I used PHP, although there are many more.
Follow the "hello world" tutorial for the framework you choose - even if you are familiar with PHP it will help you be efficient with the framework from the beginning.
> Learn to design/implement very basic design using CSS/jQuery
Google for some getting started tutorials for each language. Once you are happy with the basics, update the website you created in the framework tutorial so that it uses your new design.
> Making it scalable
Do the other two first, then come back to this. I am not suggesting you ignore it completely, but simply being aware of things like caching and optimising SQL queries is probably enough to get your first project done.
In a recent reply to someone asking how to scale I advised testing it under heavy load to see what breaks. Fix the thing that breaks, then test it until something else breaks. And so on.
"Making is scalable" is very broad topic - you need to break this down in to many small, easy to Google problems.