Based on your post, I am going to assume you are very new to programming (both system and web).
In order to create 'big things for the web', you have to not only understand how the web works, but the technologies that are used to make it happen. This requires a lot of time, patience, learning, and practice.
You said that you are currently learning javascript, and your main question seems to be whether or not you should start by learning PHP or Ruby on Rails (RoR). Here's what I'd recommend (I'm not answering your question exactly, but hopefully this helps you get a good start.):
- Learn HTML. This will take some time. You should start by going through the w3school's HTML tutorials (http://www.w3schools.com/html/default.asp), and then build some simple HTML-only web pages. Do not use any javascript, and don't use anything else. This phase should occupy one month of time (take it slow), and really learn the stuff. You should probably spend 1->3 hours a day working on it (this timeframe applies not only to this number, but to all other numbers below).
- Learn CSS. You can also go through the w3schools tutorials on this for free online. And again, spend about a month working with CSS only. Make web pages with HTML & CSS, and stylize them. Spend a lot of time on it. Read articles online. Search for more information.
- Now you can being learning javascript. Learn the core language first (one month or so), and write your own effects and web pages with it. After you've gotten good enough with it so that you feel fluent, begin learning jQuery and start the process all over again.
- Start learning Ruby, read some books, tutorials, whatever it takes. This may take a long time. Programming is not very easy to pick up for most beginners. Spend several months (or more) working on code, writing small programs, and generally learning how things work. Take some computer science classes (and / or get a CS degree) at a university near you.
- Start learning Ruby on Rails. Now that you know HTML, CSS, javascript, jQuery, and Ruby, you should feel comfortable designing flexible web applications with RoR.
There's really a lot for you to learn before you can create any programs that will do 'something big'. Most programmers spend years of their lives studying and practicing coding (in tons of languages), and still never develop any large applications.
The best advice I can give is to take your time, and just enjoy it for what it is: learning a new trade.
Best of luck.