hmm its not easy, it takes alot of EQing and even then its difficult to totally get rid of the background music. and sometimes you EQ it so much that the vocal sound quality degrades.
there is something called blue chip technology that i hear can remove vocals but i dont know how true this is.
You'll never completley remove the bg music from the vocals. However, a certain amount of EQing will get you close. Audacity has a noice removal feature which can also be used in conjunction with EQing but you'll never turn a full track into what is essentially an accapella.
Removing the instrumental from the vocal is impossible and vice versa, unless you have both instrumental and vocal seperately. You can try looking for accapellas on vinyl through websites. Thats the only best alternative or try to EQ it as much as possible.