Learn HTML and optionally other technologies such as CGI, JavaScript etc. Either teach yourself from books and sites like WDVL or look for courses. Set up a practice site and seek feedback and constructive criticism. Validate the HTML . The costs are highly variable, depending on many factors, e.g. the quality of staff allocated to it - but often the costs are grossly underestimated.
Costs arise from training, software and equipment, development time, promotion, etc.
The most difficult part for most people is realising that the web was not designed as a DTP system - trying to get HTML to layout pages exactly as designed, rather than as logical structures with informational architecture. They sometimes end up putting their message into graphics alone, creating problems for search engines, the blind, etc. The easiest is learning sufficient HTML to create something. HTML is remarkably empowering and easy to get started with, which is why it has been so succesful. But after a short time, most people focus on visual aspects such as exact layout, instead of the more abstract issues of navigation, usability, accessibility, etc.