What is the best way to convincing someone to let you buy something you really want?

Question

i am buying it with my own money just my parents

say that i am wasting my money when i really want it

Answer 1

Show them that you will use the item after you have bought it.

Answer 2

say you don't want it any more

Answer 3

Here's a thought, pay for it with your own money.  Don't expect your parents to buy you every little toy you think you need.