Supposing that I have to administer a website(changing photos,uploading others, changing texts and prices and so on ) -for a job- ,how could I do that ? What skills are required? And what programs should I use? I really need some help .

If you have to ask. .... You are NOT suitable for, or capable enough to do the job.

It really depends on the specific site. If the site is built on some kind do content management system or shopping cart you might only need to become familiar with that system. It might only be logging in to a backend for the site and finding where you can change product information like prices, etc.

If you're going to admin a website you should also be familiar with more than how to log in to a single site and change text and prices. You'll probably want an understanding of html and css and how to access files on a website. A code editor and an FTP program will help and become necessary tools. In addition to html and css you'd want to learn any technology that was used in building the specific site.

However, Chris is right in saying if you have to ask this now, you probably aren't ready to manage this site completely. If all you need to do is login to a backend system and figure out how to change a price you certainly can, but if you need to do more and understand how to edit files it'll be better to learn more first.