I want to make the clients web page more intresting than just plain text by adding diffrent kinds of blocks like "Specal offers", "Who we are", "Recent works" things like that. The problem is that I don't really know what the client could want there, and he does not know it either most times.

Any way, the question is - what do you discuss with you'r clients before building the actual web page?

First and foremost, please don’t put in filler content (content existing simply to fill the site out). You should perform a "content discovery" phase, in which the end result is a "content plan".

You start by asking three questions:
1. What does the audience expect?
2. What does the audience want?
3. What do you want the audience to want?

After you've answered those, you need to evaluate what content the client has, what content the client wants and want content can actually be available for launch to help further answer the above mentioned three questions.

Ideally, you would now begin the sitemap and wireframe process (start building the information architecture). Figure out where all of that content goes and how it is accessed -- start developing a hierarchy of content.