Web based software is not just one language, minimally, you have to get proficient with HTML, a programming language, and (if needed) a database engine.
Add on to it if you want it to look better add on CSS, or run nicer on the browser add in Javascript.
Each of those technologies takes time to learn and even more time to become adept at them.
As far as what language, that depends on what you (as a developer) will find out most proficient in, as well as the ultimate purpose of your project. Some languages/platforms are easier/harder to do certain things with.
Yes there are websites that are 100% free. Once you figure out what you want to learn, it gets easier to find the sites to help you.
Its not easy building web based software
a) Undergraduates in IT/Computer science understand the concept but may not have honed their skills enough to make something as polished.
b) They may lack the systems knowledge of the project in order to effectively write a workable solution (i.e they well know how to build a database but no idea on making one covering all the ins and outs of construction management).
It will take years before you are really proficient. Sometimes you will get lucky, but the most part is learning and try try again till you get it right.