As a graphic designer specialized in printing designs, how can I start my steps to learn web designing?

you can start with some UI experience books in front-end design after that you can dive-in UI development frameworks such XHTML , CSS , Bootstrap and JS .

You are already a designer by nature, where you worked with the media that is fixed. You already have the understanding of universal design principles. 

Web Medium is not fixed medium it varies from device to device. On web there comes device with rams, processors, storage, network latency. Just consider these factors and design accordingly. 

As far as learning curve is concerned, you just need to learn html and css to create a most usable and accessable website.

Best place to start with is The html and Css book by Jon Duckett. It's really nice piece written from people coming from any discipline to Web Design.

you need to learn the basics of the web language mainly php, java, & html

Obviously, this is one of the popular choices when it comes to specialization. Try reading The Principles of Beautiful Web Design as it has all the basics you need. Feel free to jump from one website to another as well, taking note of the design aspects that you like the most about each website you see.