My paper is accepted for a presentation in a conference; but I can not attend. Now what? I can not attend the conference and present my work. Besides they (organizers) have asked us to declare them "right away" in the case we can not attend for any reasons. Now I have two questions:

1- Will my paper lose the chance to be published in the selected papers of the conference (which is almost 20 among 100 presented ones)?  Or it still has the chance even without my presentation (e.g., for sake of its content)

2- They have announced to publish the conference details (including lecturers' names and papers) on their website in a week. Now, should I wait to see my name on their page (in order to reference specific people; e.g. an admission committee of somewhere) and then tell them I can not attend the conference? Or is it immoral and/or just telling the story (and not referring to a webpage) would suffice?

Thanks
You'd have to ask the conference organization. You should try to get somebody to present the work for you, or (just out of courtesy) tell them it won't be presented (so they can schedule talks correctly).

What will happen is up to the conference. Some I've attended just did not publish papers which weren't presented, others did publish in their proceedings regardless (you had to pay any attendance fees, at the very least). What happens with "selected papers" also varies, one conference I went to selected papers only among those presented, including in their selection the presentation and questions and answers, asking selected authors for a revision based on the above.