Like Gilson said, this gets covered almost every day. Do a search before posting. It'salso covered in this sticky: Sampling faq read before you post!!!! (removing/extracting instruments from a sample)
Short answer to your question: You cannot extract certain sounds from a track. You can try eq some stuff out, but you won't be able to remove everything else besides the vocals. If you are looking for acapella's your best bet would be buying a single that has an acapella version on it, at least 12" hip hop singles often include acapellas.
It's also possible to make an acapella if you have the instrumental version without vocals by lining it up with the version that got vocals and using phase inversion: https://www.youtube.com/watch?v=pLGZU5aWmjc