how can i learn surfacing?

Siavash,

A good place to start would be Matt Lombard's Surfacing book, which is, unfortunately, out of print. You can still buy it from Amazon for a Kindle. Cheaper to buy a Kindle and download it than to buy a paperback.

I really learned a lot from Ed Eaton's "Curvy Stuff" tutorials:

http://www.dimontegroup.com/Tutorials/SolidWorks_Tutorials.htm

These are fairly old, so there is a lot of out of date information when it comes to the specifics, but the ideas are still valid. For example, the early ones were written before the Boundary Surface existed.

Another good source is the Surfacing category on Charles Culp's website:

http://www.swtuts.com/

Jerry S.