How to build a modern usable user friendly web application?

I have a question that is not related to programming. I developed web sites for more than 3 years, but most of the time as the programmer. My biggest problem is that I don't know how to start designing the UI of a web applications that is modern and user friendly and also beautiful.

I know that my question is very general but if someone wants to know how to design a website, or use this or that color, and so on, in the best practise, what he or she has to search?

What tutorial can you suggest to someone to just design the UI on the paper or with some tools?

I don't care about implementation. I searched a lot about this but I have not yet reached a quite well and complete guide line for just designing a web site or web application that looks very user friendly.

My be I don't know what keywords I have to search regarding this.