There is no specific amount that can be used without permission; the law only says the amount should be "appropriate," and gives no further guidelines.
You can use others' protected material if you have an exemption in the law or a license from the copyright holder.
You will find that most material on websites is protected under copyright. The website owner should be able to tell you who the copyright owner is. You would then negotiate a price with the copyright owner to allow you to use their material.
You may use copyright protected material when you are the copyright holder, or when you have permission from the rightsholder or an exemption in the law. The most notable exemption is fair use or fair dealing, which allows certain limited unlicensed uses in situations such as education and commentary.
To determine if material posted online is copyrighted, look for a copyright symbol (), the word "copyright," or the phrase "all rights reserved." Additionally, check for any statements or disclaimers indicating the material is protected by copyright law. You can also search for the material in copyright databases or contact the creator for permission to use it.
It depends what kind of material and how much. There are laws protecting the fair use of copyrighted material, but lifting an entire website wholesale is plagiarism and copyright infringement.
Anyone can create a work of sufficient creativity and have it automatically protected by copyright.
In order to use copyright-protected materials that are not your own original work, you need permission from the rightsholder; this is usually in the form of a license, which can be as broad as a Creative Commons license or (more often) specific to your proposed use.
Check the terms of use statement at the bottom of the page. The material may be copyright protected.
Creative Commons material is protected by copyright, but issued with an extremely broad license allowing a variety of free uses.
If you are taking a clip from the movie, the movie itself is protected by copyright, but depending on your use, it may be covered by 17USC107. If you are using a photograph of it, the photograph would be protected by copyright, but your use may be covered by 17USC107.
The copyright holder, or anyone the copyright holder authorizes.
That would be copyright infringement if the original song was protected.