I am looking at getting a new cassette for my mountain bike ,and wondering if i need to change anything else(derailleur,shifter,chain)?
If you're getting a new cassette because it is worn, then you should at least get a new chain at the same time. You should also inspect your chainrings for wear and change them if needed. The derailleurs/shifters would only need to be changed if you get a different number of cogs on the cassette. They can wear out, but it's not usually in sync with the other items.