I bought a new domain but when I got to this domain it automatically takes me to the another domain within my same hosting why?
You've told us the start and the end of the story. What about filling in some bits in between. You bought the domain... then what? Its redirecting to another domain on your own hosting. So Did you change the DNS Correctly? Have you setup the hosting account on the host? Have you got a redirect setup in the hosting account for the new domain to point to this other domain? After you bought the domain, what did you do?
Ok so you only changed the nameserver. It be good if you either put the URL in question on here or you can PM me. That way I'll be able to get a better handle on what you may not have done than playing 20 questions on here! Then we can get you up and running. Cheers Tim