How do I learn to be a professional WebDesigner ...

Well, I don't know what you mean by professional and what other work experience you have. In my opinion here are 2 parts to this question - one is "how to be a graphic designer?" and the second is "how to be a web designer?" In my experience - the first one is harder to lean then the other.

The web part of this is really technical, I didn't know anything about web design 6 months ago, I got a couple of books and "dreamweaver" and leared how to build  simple sites (one that just involved HTML code and don't have other components to them).

As for style - I've learn graphic design for 4 years in a design school. I think it was a too long program, but it did change the way I look at things and solve problems. There are a lot of school that teach web design or graphic design. I think it's also work for some people to just look on a lot of sites and books and see what they like and why, personally, I don't think I could learn it without teachers and other students.

I guess it all depends on what kind of things you are good at and what ways you are comfortable learning anything else.

Methods I can think of are:

1. Go to web design school or take a few courses.

2. buy programs and a book and teach yourself.

3. Get a privet teacher to give you classes

4. if you are more technically oriented, you can research online and just learn to code HTML \ XHTML on your own, without using dreamweaver or other programs.

I guess each method is better for different people.

good luck! 

lilly answered your question insightfully though I do not agree on a couple of issues. Graphic design is not harder than web design. I tend to think the opposite, because to be a web designer you have to be a graphic designer, since web design is only a form of graphic design applied to a different medium. What makes it more complex is that you need to know a lot of related technologies, such as Javascript, CSS, also basic html, which is very clunky, and now since new technologies are emerging every day you need really to be working in a design firm and specialize in one aspect like designing the visuals in photoshop and letting programmer flesh it out. But if you're a freelance designer it can be very challenging. You cannot know photoshop, illustrator, dreamweaver, fireworks, flash, javascript, CSS, html, cold fusion, CMS, PhP, java.. and the list goes on and on..

Though I agree that 4 years of college is too much to study graphic design, I do think it would not be bad to consider going to school to study web design. At least they will give you good orientation and teach you marketable skills without you being too confused. 

School, college and a brain

short and sweet

Most community colleges, and even a few senior centers and high schools, offer classes on web design.  If you're looking for basic information you can take the courses on a no credit basis for personal enrichment, but most offer a 1 to 2 year certificate course that's more in depth.

The best way is "the practic" and learning new staff and codes from books and courses. you can focus in onle one area as html, css, flash like this web design.