If you want to develop a plugin for CMSimple_XH, but don't know how to do it, you've just found the right place! The information in the following chapters might be useful, even if you've already developed CMSimple(_XH) plugins.
BTW: do not forget, that this is a WIKI!
That's all! If you do not fulfill all of this requirements, it's a waste of time to go on with this tutorial! So:
Please note that www.w3schools is neither affiliated to the W3C, nor that its tutorials are highly recommendable, as they're lacking many important details and might even offer incorrect information. I've linked them, as they offer a consistent look and feel for the tutorials on the different topics and are suited for a quick start. For a good reference about HTML and CSS consult the specs. For PHP it's recommendable to read PHP – The Right Way and to use the documentation on php.net. Regarding javascript good information can be found on MDN and of course in the specs.