How does plagiarism work for samples?

IE: If I were to take like a 10 second clip from a movie or video game or something (does it depend on the source you take it from?) how do I go about giving credit to it, or is this sort of thing only allowed if you somehow contact whoever has rights to whatever it is you would like to use a sample of?

Thanks for the help and if my question is vague or isn't clear enough or something let me know.

this might help http://en.wikipedia.org/wiki/Fair_use

For the vast majority of commercial music/movies, yes, you'd need to get permission. Having said that, if you're not going to sell it or release it in any major sort of way, I wouldn't bother. The worst that will probably happen is that the file gets taken down.